diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Feature.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Feature.kt
index ea22bdfb..cdbd8d9e 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Feature.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Feature.kt
@@ -1,6 +1,8 @@
 package io.github.dellisd.spatialk.geojson
 
 import io.github.dellisd.spatialk.geojson.serialization.FeatureSerializer
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembers
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembersJsonProps
 import io.github.dellisd.spatialk.geojson.serialization.idProp
 import io.github.dellisd.spatialk.geojson.serialization.jsonProp
 import io.github.dellisd.spatialk.geojson.serialization.toBbox
@@ -36,7 +38,8 @@ class Feature(
     val geometry: Geometry?,
     properties: Map<String, JsonElement> = emptyMap(),
     val id: String? = null,
-    override val bbox: BoundingBox? = null
+    override val bbox: BoundingBox? = null,
+    override val foreignMembers: Map<String, JsonElement> = emptyMap()
 ) : GeoJson {
     private val _properties: MutableMap<String, JsonElement> = properties.toMutableMap()
     val properties: Map<String, JsonElement> get() = _properties
@@ -86,6 +89,7 @@ class Feature(
         if (id != other.id) return false
         if (bbox != other.bbox) return false
         if (_properties != other._properties) return false
+        if (foreignMembers != other.foreignMembers) return false
 
         return true
     }
@@ -95,6 +99,7 @@ class Feature(
         result = 31 * result + (id?.hashCode() ?: 0)
         result = 31 * result + (bbox?.hashCode() ?: 0)
         result = 31 * result + _properties.hashCode()
+        result = 31 * result + foreignMembers.hashCode()
         return result
     }
 
@@ -102,6 +107,7 @@ class Feature(
     operator fun component2() = properties
     operator fun component3() = id
     operator fun component4() = bbox
+    operator fun component5() = foreignMembers
 
     override fun toString(): String = json()
 
@@ -113,16 +119,18 @@ class Feature(
                     JsonElement.serializer()
                 ), properties
             )
-        }}"""
+        }${foreignMembersJsonProps()}}"""
 
     fun copy(
         geometry: Geometry? = this.geometry,
         properties: Map<String, JsonElement> = this.properties,
         id: String? = this.id,
-        bbox: BoundingBox? = this.bbox
-    ): Feature = Feature(geometry, properties, id, bbox)
+        bbox: BoundingBox? = this.bbox,
+        foreignMembers: Map<String, JsonElement> = this.foreignMembers
+    ): Feature = Feature(geometry, properties, id, bbox, foreignMembers)
 
     companion object {
+
         @JvmStatic
         fun fromJson(json: String): Feature = fromJson(Json.decodeFromString(JsonObject.serializer(), json))
 
@@ -145,7 +153,10 @@ class Feature(
             val geom = json["geometry"]?.jsonObject
             val geometry: Geometry? = if (geom != null) Geometry.fromJson(geom) else null
 
-            return Feature(geometry, json["properties"]?.jsonObject ?: emptyMap(), id, bbox)
+            val properties = json["properties"]?.jsonObject ?: emptyMap()
+            val foreignMembers = json.foreignMembers()
+
+            return Feature(geometry, properties, id, bbox, foreignMembers)
         }
     }
 }
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/FeatureCollection.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/FeatureCollection.kt
index 6fb43fa3..a1b5cdb4 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/FeatureCollection.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/FeatureCollection.kt
@@ -1,11 +1,14 @@
 package io.github.dellisd.spatialk.geojson
 
 import io.github.dellisd.spatialk.geojson.serialization.FeatureCollectionSerializer
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembers
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembersJsonProps
 import io.github.dellisd.spatialk.geojson.serialization.jsonJoin
 import io.github.dellisd.spatialk.geojson.serialization.jsonProp
 import io.github.dellisd.spatialk.geojson.serialization.toBbox
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.jsonArray
 import kotlinx.serialization.json.jsonObject
@@ -24,10 +27,15 @@ import kotlin.jvm.JvmStatic
 @Serializable(with = FeatureCollectionSerializer::class)
 class FeatureCollection(
     val features: List<Feature> = emptyList(),
-    override val bbox: BoundingBox? = null
+    override val bbox: BoundingBox? = null,
+    override val foreignMembers: Map<String, JsonElement> = emptyMap()
 ) : Collection<Feature> by features, GeoJson {
 
-    constructor(vararg features: Feature, bbox: BoundingBox? = null) : this(features.toMutableList(), bbox)
+    constructor(
+        vararg features: Feature,
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(features.toMutableList(), bbox, foreignMembers)
 
     override fun equals(other: Any?): Boolean {
         if (this === other) return true
@@ -37,6 +45,7 @@ class FeatureCollection(
 
         if (features != other.features) return false
         if (bbox != other.bbox) return false
+        if (foreignMembers != other.foreignMembers) return false
 
         return true
     }
@@ -44,18 +53,22 @@ class FeatureCollection(
     override fun hashCode(): Int {
         var result = features.hashCode()
         result = 31 * result + (bbox?.hashCode() ?: 0)
+        result = 31 * result + foreignMembers.hashCode()
         return result
     }
 
     override fun toString(): String = json()
 
+    @Suppress("MaxLineLength")
     override fun json(): String =
-        """{"type":"FeatureCollection",${bbox.jsonProp()}"features":${features.jsonJoin { it.json() }}}"""
+        """{"type":"FeatureCollection",${bbox.jsonProp()}"features":${features.jsonJoin { it.json() }}${foreignMembersJsonProps()}}"""
 
     operator fun component1(): List<Feature> = features
     operator fun component2(): BoundingBox? = bbox
+    operator fun component3(): Map<String, JsonElement> = foreignMembers
 
     companion object {
+
         @JvmStatic
         public fun fromJson(json: String): FeatureCollection =
             fromJson(Json.decodeFromString(JsonObject.serializer(), json))
@@ -75,8 +88,9 @@ class FeatureCollection(
 
             val bbox = json["bbox"]?.jsonArray?.toBbox()
             val features = json.getValue("features").jsonArray.map { Feature.fromJson(it.jsonObject) }
+            val foreignMembers = json.foreignMembers()
 
-            return FeatureCollection(features, bbox)
+            return FeatureCollection(features, bbox, foreignMembers)
         }
     }
 }
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/GeoJson.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/GeoJson.kt
index cb1acb43..73cd41b5 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/GeoJson.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/GeoJson.kt
@@ -1,5 +1,7 @@
 package io.github.dellisd.spatialk.geojson
 
+import kotlinx.serialization.json.JsonElement
+
 /**
  * A GeoJSON object represents a [Geometry], [Feature], or [collection of Features][FeatureCollection].
  *
@@ -7,6 +9,7 @@ package io.github.dellisd.spatialk.geojson
  */
 interface GeoJson {
     val bbox: BoundingBox?
+    val foreignMembers: Map<String, JsonElement>
 
     /**
      * Gets a JSON representation of this object.
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Geometry.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Geometry.kt
index d6ce39b7..68e1fad1 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Geometry.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Geometry.kt
@@ -3,13 +3,38 @@ package io.github.dellisd.spatialk.geojson
 import io.github.dellisd.spatialk.geojson.serialization.GeometrySerializer
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.jsonPrimitive
 import kotlin.jvm.JvmStatic
 
 @Serializable(with = GeometrySerializer::class)
-sealed class Geometry protected constructor() : GeoJson {
-    abstract override val bbox: BoundingBox?
+sealed class Geometry protected constructor(
+    override val bbox: BoundingBox? = null,
+    override val foreignMembers: Map<String, JsonElement> = emptyMap()
+) : GeoJson {
+
+    protected abstract val coordinatesOrGeometries: Any
+
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other == null || this::class != other::class) return false
+
+        other as Geometry
+
+        if (coordinatesOrGeometries != other.coordinatesOrGeometries) return false
+        if (bbox != other.bbox) return false
+        if (foreignMembers != other.foreignMembers) return false
+
+        return true
+    }
+
+    override fun hashCode(): Int {
+        var result = coordinatesOrGeometries.hashCode()
+        result = 31 * result + (bbox?.hashCode() ?: 0)
+        result = 31 * result + foreignMembers.hashCode()
+        return result
+    }
 
     override fun toString(): String = json()
 
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/GeometryCollection.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/GeometryCollection.kt
index 4af24319..29b58e25 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/GeometryCollection.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/GeometryCollection.kt
@@ -1,11 +1,14 @@
 package io.github.dellisd.spatialk.geojson
 
 import io.github.dellisd.spatialk.geojson.serialization.GeometrySerializer
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembers
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembersJsonProps
 import io.github.dellisd.spatialk.geojson.serialization.jsonJoin
 import io.github.dellisd.spatialk.geojson.serialization.jsonProp
 import io.github.dellisd.spatialk.geojson.serialization.toBbox
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.jsonArray
 import kotlinx.serialization.json.jsonObject
@@ -17,31 +20,21 @@ import kotlin.jvm.JvmStatic
 @Serializable(with = GeometrySerializer::class)
 class GeometryCollection @JvmOverloads constructor(
     val geometries: List<Geometry>,
-    override val bbox: BoundingBox? = null
-) : Geometry(), Collection<Geometry> by geometries {
-    @JvmOverloads
-    constructor(vararg geometries: Geometry, bbox: BoundingBox? = null) : this(geometries.toList(), bbox)
-
-    override fun equals(other: Any?): Boolean {
-        if (this === other) return true
-        if (other == null || this::class != other::class) return false
-
-        other as GeometryCollection
-
-        if (geometries != other.geometries) return false
-        if (bbox != other.bbox) return false
+    bbox: BoundingBox? = null,
+    foreignMembers: Map<String, JsonElement> = emptyMap()
+) : Geometry(bbox, foreignMembers), Collection<Geometry> by geometries {
+    override val coordinatesOrGeometries get() = geometries
 
-        return true
-    }
-
-    override fun hashCode(): Int {
-        var result = geometries.hashCode()
-        result = 31 * result + (bbox?.hashCode() ?: 0)
-        return result
-    }
+    @JvmOverloads
+    constructor(
+        vararg geometries: Geometry,
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(geometries.toList(), bbox, foreignMembers)
 
+    @Suppress("MaxLineLength")
     override fun json(): String =
-        """{"type":"GeometryCollection",${bbox.jsonProp()}"geometries":${geometries.jsonJoin { it.json() }}}"""
+        """{"type":"GeometryCollection",${bbox.jsonProp()}"geometries":${geometries.jsonJoin { it.json() }}${foreignMembersJsonProps()}}"""
 
     companion object {
         @JvmStatic
@@ -66,8 +59,9 @@ class GeometryCollection @JvmOverloads constructor(
             }
 
             val bbox = json["bbox"]?.jsonArray?.toBbox()
+            val foreignMembers = json.foreignMembers()
 
-            return GeometryCollection(geometries, bbox)
+            return GeometryCollection(geometries, bbox, foreignMembers)
         }
     }
 }
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/LineString.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/LineString.kt
index a4380119..b3f96aac 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/LineString.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/LineString.kt
@@ -1,12 +1,15 @@
 package io.github.dellisd.spatialk.geojson
 
 import io.github.dellisd.spatialk.geojson.serialization.GeometrySerializer
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembers
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembersJsonProps
 import io.github.dellisd.spatialk.geojson.serialization.jsonJoin
 import io.github.dellisd.spatialk.geojson.serialization.jsonProp
 import io.github.dellisd.spatialk.geojson.serialization.toBbox
 import io.github.dellisd.spatialk.geojson.serialization.toPosition
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.jsonArray
 import kotlinx.serialization.json.jsonPrimitive
@@ -17,41 +20,32 @@ import kotlin.jvm.JvmStatic
 @Serializable(with = GeometrySerializer::class)
 class LineString @JvmOverloads constructor(
     val coordinates: List<Position>,
-    override val bbox: BoundingBox? = null
-) : Geometry() {
+    bbox: BoundingBox? = null,
+    foreignMembers: Map<String, JsonElement> = emptyMap()
+) : Geometry(bbox, foreignMembers) {
+    override val coordinatesOrGeometries get() = coordinates
+
     @JvmOverloads
-    constructor(vararg coordinates: Position, bbox: BoundingBox? = null) : this(coordinates.toList(), bbox)
+    constructor(
+        vararg coordinates: Position,
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(coordinates.toList(), bbox, foreignMembers)
 
     @JvmOverloads
     constructor(
         coordinates: Array<DoubleArray>,
-        bbox: BoundingBox? = null
-    ) : this(coordinates.map(::Position), bbox)
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(coordinates.map(::Position), bbox, foreignMembers)
 
     init {
         require(coordinates.size >= 2) { "LineString must have at least two positions" }
     }
 
-    override fun equals(other: Any?): Boolean {
-        if (this === other) return true
-        if (other == null || this::class != other::class) return false
-
-        other as LineString
-
-        if (coordinates != other.coordinates) return false
-        if (bbox != other.bbox) return false
-
-        return true
-    }
-
-    override fun hashCode(): Int {
-        var result = coordinates.hashCode()
-        result = 31 * result + (bbox?.hashCode() ?: 0)
-        return result
-    }
-
+    @Suppress("MaxLineLength")
     override fun json(): String =
-        """{"type":"LineString",${bbox.jsonProp()}"coordinates":${coordinates.jsonJoin(transform = Position::json)}}"""
+        """{"type":"LineString",${bbox.jsonProp()}"coordinates":${coordinates.jsonJoin(transform = Position::json)}${foreignMembersJsonProps()}}"""
 
     companion object {
         @JvmStatic
@@ -72,8 +66,9 @@ class LineString @JvmOverloads constructor(
 
             val coords = json.getValue("coordinates").jsonArray.map { it.jsonArray.toPosition() }
             val bbox = json["bbox"]?.jsonArray?.toBbox()
+            val foreignMembers = json.foreignMembers()
 
-            return LineString(coords, bbox)
+            return LineString(coords, bbox, foreignMembers)
         }
     }
 }
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiLineString.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiLineString.kt
index e88ce4f0..9f3628a7 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiLineString.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiLineString.kt
@@ -1,12 +1,15 @@
 package io.github.dellisd.spatialk.geojson
 
 import io.github.dellisd.spatialk.geojson.serialization.GeometrySerializer
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembers
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembersJsonProps
 import io.github.dellisd.spatialk.geojson.serialization.jsonJoin
 import io.github.dellisd.spatialk.geojson.serialization.jsonProp
 import io.github.dellisd.spatialk.geojson.serialization.toBbox
 import io.github.dellisd.spatialk.geojson.serialization.toPosition
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.jsonArray
 import kotlinx.serialization.json.jsonPrimitive
@@ -17,16 +20,24 @@ import kotlin.jvm.JvmStatic
 @Serializable(with = GeometrySerializer::class)
 class MultiLineString @JvmOverloads constructor(
     val coordinates: List<List<Position>>,
-    override val bbox: BoundingBox? = null
-) : Geometry() {
+    bbox: BoundingBox? = null,
+    foreignMembers: Map<String, JsonElement> = emptyMap()
+) : Geometry(bbox, foreignMembers) {
+    override val coordinatesOrGeometries get() = coordinates
+
     @JvmOverloads
-    constructor(vararg coordinates: List<Position>, bbox: BoundingBox? = null) : this(coordinates.toList(), bbox)
+    constructor(
+        vararg coordinates: List<Position>,
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(coordinates.toList(), bbox, foreignMembers)
 
     @JvmOverloads
     constructor(
         coordinates: Array<Array<DoubleArray>>,
-        bbox: BoundingBox? = null
-    ) : this(coordinates.map { it.map(::Position) }, bbox)
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(coordinates.map { it.map(::Position) }, bbox, foreignMembers)
 
     init {
         coordinates.forEach { line ->
@@ -34,30 +45,12 @@ class MultiLineString @JvmOverloads constructor(
         }
     }
 
-    override fun equals(other: Any?): Boolean {
-        if (this === other) return true
-        if (other == null || this::class != other::class) return false
-
-        other as MultiLineString
-
-        if (coordinates != other.coordinates) return false
-        if (bbox != other.bbox) return false
-
-        return true
-    }
-
-    override fun hashCode(): Int {
-        var result = coordinates.hashCode()
-        result = 31 * result + (bbox?.hashCode() ?: 0)
-        return result
-    }
-
     override fun json(): String =
         """{"type":"MultiLineString",${bbox.jsonProp()}"coordinates":${
             coordinates.jsonJoin {
                 it.jsonJoin(transform = Position::json)
             }
-        }}"""
+        }${foreignMembersJsonProps()}}"""
 
     companion object {
         @JvmStatic
@@ -80,8 +73,9 @@ class MultiLineString @JvmOverloads constructor(
             val coords =
                 json.getValue("coordinates").jsonArray.map { line -> line.jsonArray.map { it.jsonArray.toPosition() } }
             val bbox = json["bbox"]?.jsonArray?.toBbox()
+            val foreignMembers = json.foreignMembers()
 
-            return MultiLineString(coords, bbox)
+            return MultiLineString(coords, bbox, foreignMembers)
         }
     }
 }
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiPoint.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiPoint.kt
index e02e6305..5895d77f 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiPoint.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiPoint.kt
@@ -1,12 +1,15 @@
 package io.github.dellisd.spatialk.geojson
 
 import io.github.dellisd.spatialk.geojson.serialization.GeometrySerializer
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembers
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembersJsonProps
 import io.github.dellisd.spatialk.geojson.serialization.jsonJoin
 import io.github.dellisd.spatialk.geojson.serialization.jsonProp
 import io.github.dellisd.spatialk.geojson.serialization.toBbox
 import io.github.dellisd.spatialk.geojson.serialization.toPosition
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.jsonArray
 import kotlinx.serialization.json.jsonPrimitive
@@ -17,37 +20,28 @@ import kotlin.jvm.JvmStatic
 @Serializable(with = GeometrySerializer::class)
 class MultiPoint @JvmOverloads constructor(
     val coordinates: List<Position>,
-    override val bbox: BoundingBox? = null
-) : Geometry() {
+    bbox: BoundingBox? = null,
+    foreignMembers: Map<String, JsonElement> = emptyMap()
+) : Geometry(bbox, foreignMembers) {
+    override val coordinatesOrGeometries get() = coordinates
+
     @JvmOverloads
-    constructor(vararg coordinates: Position, bbox: BoundingBox? = null) : this(coordinates.toList(), bbox)
+    constructor(
+        vararg coordinates: Position,
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(coordinates.toList(), bbox, foreignMembers)
 
     @JvmOverloads
     constructor(
         coordinates: Array<DoubleArray>,
-        bbox: BoundingBox? = null
-    ) : this(coordinates.map(::Position), bbox)
-
-    override fun equals(other: Any?): Boolean {
-        if (this === other) return true
-        if (other == null || this::class != other::class) return false
-
-        other as MultiPoint
-
-        if (coordinates != other.coordinates) return false
-        if (bbox != other.bbox) return false
-
-        return true
-    }
-
-    override fun hashCode(): Int {
-        var result = coordinates.hashCode()
-        result = 31 * result + (bbox?.hashCode() ?: 0)
-        return result
-    }
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(coordinates.map(::Position), bbox, foreignMembers)
 
+    @Suppress("MaxLineLength")
     override fun json(): String =
-        """{"type":"MultiPoint",${bbox.jsonProp()}"coordinates":${coordinates.jsonJoin(transform = Position::json)}}"""
+        """{"type":"MultiPoint",${bbox.jsonProp()}"coordinates":${coordinates.jsonJoin(transform = Position::json)}${foreignMembersJsonProps()}}"""
 
     companion object {
         @JvmStatic
@@ -68,8 +62,9 @@ class MultiPoint @JvmOverloads constructor(
 
             val coords = json.getValue("coordinates").jsonArray.map { it.jsonArray.toPosition() }
             val bbox = json["bbox"]?.jsonArray?.toBbox()
+            val foreignMembers = json.foreignMembers()
 
-            return MultiPoint(coords, bbox)
+            return MultiPoint(coords, bbox, foreignMembers)
         }
     }
 }
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiPolygon.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiPolygon.kt
index 957dc5b6..84affd24 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiPolygon.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/MultiPolygon.kt
@@ -1,12 +1,15 @@
 package io.github.dellisd.spatialk.geojson
 
 import io.github.dellisd.spatialk.geojson.serialization.GeometrySerializer
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembers
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembersJsonProps
 import io.github.dellisd.spatialk.geojson.serialization.jsonJoin
 import io.github.dellisd.spatialk.geojson.serialization.jsonProp
 import io.github.dellisd.spatialk.geojson.serialization.toBbox
 import io.github.dellisd.spatialk.geojson.serialization.toPosition
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.jsonArray
 import kotlinx.serialization.json.jsonPrimitive
@@ -17,34 +20,24 @@ import kotlin.jvm.JvmStatic
 @Serializable(with = GeometrySerializer::class)
 class MultiPolygon @JvmOverloads constructor(
     val coordinates: List<List<List<Position>>>,
-    override val bbox: BoundingBox? = null
-) : Geometry() {
+    bbox: BoundingBox? = null,
+    foreignMembers: Map<String, JsonElement> = emptyMap()
+) : Geometry(bbox, foreignMembers) {
+    override val coordinatesOrGeometries get() = coordinates
+
     @JvmOverloads
-    constructor(vararg coordinates: List<List<Position>>, bbox: BoundingBox? = null) : this(coordinates.toList(), bbox)
+    constructor(
+        vararg coordinates: List<List<Position>>,
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(coordinates.toList(), bbox, foreignMembers)
 
     @JvmOverloads
     constructor(
         coordinates: Array<Array<Array<DoubleArray>>>,
-        bbox: BoundingBox? = null
-    ) : this(coordinates.map { ring -> ring.map { it.map(::Position) } }, bbox)
-
-    override fun equals(other: Any?): Boolean {
-        if (this === other) return true
-        if (other == null || this::class != other::class) return false
-
-        other as MultiPolygon
-
-        if (coordinates != other.coordinates) return false
-        if (bbox != other.bbox) return false
-
-        return true
-    }
-
-    override fun hashCode(): Int {
-        var result = coordinates.hashCode()
-        result = 31 * result + (bbox?.hashCode() ?: 0)
-        return result
-    }
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(coordinates.map { ring -> ring.map { it.map(::Position) } }, bbox, foreignMembers)
 
     override fun json(): String =
         """{"type":"MultiPolygon",${bbox.jsonProp()}"coordinates":${
@@ -53,7 +46,7 @@ class MultiPolygon @JvmOverloads constructor(
                     it.jsonJoin(transform = Position::json)
                 }
             }
-        }}"""
+        }${foreignMembersJsonProps()}}"""
 
     companion object {
         @JvmStatic
@@ -78,8 +71,9 @@ class MultiPolygon @JvmOverloads constructor(
                     polygon.jsonArray.map { ring -> ring.jsonArray.map { it.jsonArray.toPosition() } }
                 }
             val bbox = json["bbox"]?.jsonArray?.toBbox()
+            val foreignMembers = json.foreignMembers()
 
-            return MultiPolygon(coords, bbox)
+            return MultiPolygon(coords, bbox, foreignMembers)
         }
     }
 }
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Point.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Point.kt
index ad86d22e..5bba25a1 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Point.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Point.kt
@@ -1,11 +1,14 @@
 package io.github.dellisd.spatialk.geojson
 
 import io.github.dellisd.spatialk.geojson.serialization.GeometrySerializer
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembers
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembersJsonProps
 import io.github.dellisd.spatialk.geojson.serialization.jsonProp
 import io.github.dellisd.spatialk.geojson.serialization.toBbox
 import io.github.dellisd.spatialk.geojson.serialization.toPosition
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.jsonArray
 import kotlinx.serialization.json.jsonPrimitive
@@ -14,29 +17,22 @@ import kotlin.jvm.JvmStatic
 
 @Suppress("SERIALIZER_TYPE_INCOMPATIBLE")
 @Serializable(with = GeometrySerializer::class)
-class Point @JvmOverloads constructor(val coordinates: Position, override val bbox: BoundingBox? = null) : Geometry() {
-    @JvmOverloads
-    constructor(coordinates: DoubleArray, bbox: BoundingBox? = null) : this(Position(coordinates), bbox)
-
-    override fun equals(other: Any?): Boolean {
-        if (this === other) return true
-        if (other == null || this::class != other::class) return false
-
-        other as Point
-
-        if (coordinates != other.coordinates) return false
-        if (bbox != other.bbox) return false
+class Point @JvmOverloads constructor(
+    val coordinates: Position,
+    bbox: BoundingBox? = null,
+    foreignMembers: Map<String, JsonElement> = emptyMap()
+) : Geometry(bbox, foreignMembers) {
+    override val coordinatesOrGeometries get() = coordinates
 
-        return true
-    }
-
-    override fun hashCode(): Int {
-        var result = coordinates.hashCode()
-        result = 31 * result + (bbox?.hashCode() ?: 0)
-        return result
-    }
+    @JvmOverloads
+    constructor(
+        coordinates: DoubleArray,
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(Position(coordinates), bbox, foreignMembers)
 
-    override fun json(): String = """{"type":"Point",${bbox.jsonProp()}"coordinates":${coordinates.json()}}"""
+    @Suppress("MaxLineLength")
+    override fun json(): String = """{"type":"Point",${bbox.jsonProp()}"coordinates":${coordinates.json()}${foreignMembersJsonProps()}}"""
 
     companion object {
         @JvmStatic
@@ -57,8 +53,9 @@ class Point @JvmOverloads constructor(val coordinates: Position, override val bb
 
             val coords = json.getValue("coordinates").jsonArray.toPosition()
             val bbox = json["bbox"]?.jsonArray?.toBbox()
+            val foreignMembers = json.foreignMembers()
 
-            return Point(coords, bbox)
+            return Point(coords, bbox, foreignMembers)
         }
     }
 }
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Polygon.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Polygon.kt
index 1cb50819..54408e87 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Polygon.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/Polygon.kt
@@ -1,12 +1,15 @@
 package io.github.dellisd.spatialk.geojson
 
 import io.github.dellisd.spatialk.geojson.serialization.GeometrySerializer
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembers
+import io.github.dellisd.spatialk.geojson.serialization.foreignMembersJsonProps
 import io.github.dellisd.spatialk.geojson.serialization.jsonJoin
 import io.github.dellisd.spatialk.geojson.serialization.jsonProp
 import io.github.dellisd.spatialk.geojson.serialization.toBbox
 import io.github.dellisd.spatialk.geojson.serialization.toPosition
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.jsonArray
 import kotlinx.serialization.json.jsonPrimitive
@@ -17,39 +20,29 @@ import kotlin.jvm.JvmStatic
 @Serializable(with = GeometrySerializer::class)
 class Polygon @JvmOverloads constructor(
     val coordinates: List<List<Position>>,
-    override val bbox: BoundingBox? = null
-) : Geometry() {
+    bbox: BoundingBox? = null,
+    foreignMembers: Map<String, JsonElement> = emptyMap()
+) : Geometry(bbox, foreignMembers) {
+    override val coordinatesOrGeometries get() = coordinates
+
     @JvmOverloads
-    constructor(vararg coordinates: List<Position>, bbox: BoundingBox? = null) : this(coordinates.toList(), bbox)
+    constructor(
+        vararg coordinates: List<Position>,
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(coordinates.toList(), bbox, foreignMembers)
 
     @JvmOverloads
     constructor(
         coordinates: Array<Array<DoubleArray>>,
-        bbox: BoundingBox? = null
-    ) : this(coordinates.map { it.map(::Position) }, bbox)
-
-    override fun equals(other: Any?): Boolean {
-        if (this === other) return true
-        if (other == null || this::class != other::class) return false
-
-        other as Polygon
-
-        if (coordinates != other.coordinates) return false
-        if (bbox != other.bbox) return false
-
-        return true
-    }
-
-    override fun hashCode(): Int {
-        var result = coordinates.hashCode()
-        result = 31 * result + (bbox?.hashCode() ?: 0)
-        return result
-    }
+        bbox: BoundingBox? = null,
+        foreignMembers: Map<String, JsonElement> = emptyMap()
+    ) : this(coordinates.map { it.map(::Position) }, bbox, foreignMembers)
 
     override fun json(): String =
         """{"type":"Polygon",${bbox.jsonProp()}"coordinates":${
             coordinates.jsonJoin { it.jsonJoin(transform = Position::json) }
-        }}"""
+        }${foreignMembersJsonProps()}}"""
 
     companion object {
         @JvmStatic
@@ -71,8 +64,9 @@ class Polygon @JvmOverloads constructor(
             val coords =
                 json.getValue("coordinates").jsonArray.map { line -> line.jsonArray.map { it.jsonArray.toPosition() } }
             val bbox = json["bbox"]?.jsonArray?.toBbox()
+            val foreignMembers = json.foreignMembers()
 
-            return Polygon(coords, bbox)
+            return Polygon(coords, bbox, foreignMembers)
         }
     }
 }
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/FeatureCollectionDsl.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/FeatureCollectionDsl.kt
index 970f0e65..fb3a9783 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/FeatureCollectionDsl.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/FeatureCollectionDsl.kt
@@ -6,27 +6,38 @@ import io.github.dellisd.spatialk.geojson.BoundingBox
 import io.github.dellisd.spatialk.geojson.Feature
 import io.github.dellisd.spatialk.geojson.FeatureCollection
 import io.github.dellisd.spatialk.geojson.Geometry
+import kotlinx.serialization.json.JsonElement
 import kotlin.jvm.JvmName
 
 @GeoJsonDsl
 class FeatureCollectionDsl(
     private val features: MutableList<Feature> = mutableListOf(),
-    var bbox: BoundingBox? = null
+    var bbox: BoundingBox? = null,
+    val foreignMembers: MutableMap<String, JsonElement> = mutableMapOf()
 ) {
     operator fun Feature.unaryPlus() {
         features.add(this)
     }
 
     fun create(): FeatureCollection =
-        FeatureCollection(features, bbox)
+        FeatureCollection(features, bbox, foreignMembers)
 
     fun feature(
         geometry: Geometry? = null,
         id: String? = null,
         bbox: BoundingBox? = null,
+        foreignMembers: ForeignMembersBuilder.() -> Unit = {},
         properties: PropertiesBuilder.() -> Unit = {}
     ) {
-        +Feature(geometry, PropertiesBuilder().apply(properties).build(), id, bbox)
+        +io.github.dellisd.spatialk.geojson.dsl.feature(geometry = geometry,
+                                                        id = id,
+                                                        bbox = bbox,
+                                                        foreignMembers = foreignMembers,
+                                                        properties = properties)
+    }
+
+    fun foreignMembers(foreignMembers: ForeignMembersBuilder.() -> Unit = {}) {
+        this.foreignMembers.putAll(ForeignMembersBuilder().apply(foreignMembers).build())
     }
 }
 
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/FeatureDsl.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/FeatureDsl.kt
index a4af667e..1c7732c6 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/FeatureDsl.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/FeatureDsl.kt
@@ -38,5 +38,11 @@ inline fun feature(
     geometry: Geometry? = null,
     id: String? = null,
     bbox: BoundingBox? = null,
+    foreignMembers: ForeignMembersBuilder.() -> Unit = {},
     properties: PropertiesBuilder.() -> Unit = {}
-) = Feature(geometry, PropertiesBuilder().apply(properties).build(), id, bbox)
+) = Feature(
+    geometry = geometry,
+    properties = PropertiesBuilder().apply(properties).build(),
+    id = id,
+    bbox = bbox,
+    foreignMembers = ForeignMembersBuilder().apply(foreignMembers).build())
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/ForeignMembersBuilder.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/ForeignMembersBuilder.kt
new file mode 100644
index 00000000..102c4288
--- /dev/null
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/ForeignMembersBuilder.kt
@@ -0,0 +1,27 @@
+package io.github.dellisd.spatialk.geojson.dsl
+
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonPrimitive
+
+@GeoJsonDsl
+class ForeignMembersBuilder {
+    private val foreignMembers = mutableMapOf<String, JsonElement>()
+
+    fun put(key: String, value: String?) {
+        foreignMembers[key] = JsonPrimitive(value)
+    }
+
+    fun put(key: String, value: Number?) {
+        foreignMembers[key] = JsonPrimitive(value)
+    }
+
+    fun put(key: String, value: Boolean?) {
+        foreignMembers[key] = JsonPrimitive(value)
+    }
+
+    fun put(key: String, value: JsonElement) {
+        foreignMembers[key] = value
+    }
+
+    fun build(): Map<String, JsonElement> = foreignMembers
+}
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/GeometryDsl.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/GeometryDsl.kt
index 9fc9fad9..6159ae56 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/GeometryDsl.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/dsl/GeometryDsl.kt
@@ -12,17 +12,25 @@ import io.github.dellisd.spatialk.geojson.MultiPoint
 import io.github.dellisd.spatialk.geojson.MultiPolygon
 import io.github.dellisd.spatialk.geojson.Point
 import io.github.dellisd.spatialk.geojson.Polygon
+import kotlinx.serialization.json.JsonElement
 import kotlin.jvm.JvmName
 
 @GeoJsonDsl
-abstract class GeometryDsl<T : Geometry> protected constructor(var bbox: BoundingBox? = null) {
+abstract class GeometryDsl<T : Geometry> protected constructor(
+    var bbox: BoundingBox? = null,
+    val foreignMembers: MutableMap<String, JsonElement> = mutableMapOf()
+) {
     abstract fun create(): T
+
+    fun foreignMembers(foreignMembers: ForeignMembersBuilder.() -> Unit = {}) {
+        this.foreignMembers.putAll(ForeignMembersBuilder().apply(foreignMembers).build())
+    }
 }
 
 @GeoJsonDsl
 class PointDsl(private var coordinates: Position) : GeometryDsl<Point>() {
     override fun create(): Point =
-        Point(coordinates, bbox)
+        Point(coordinates, bbox, foreignMembers)
 }
 
 @GeoJsonDsl
@@ -43,7 +51,7 @@ inline fun point(
 @GeoJsonDsl
 class MultiPointDsl(private val points: MutableList<Position> = mutableListOf()) : GeometryDsl<MultiPoint>() {
     override fun create(): MultiPoint =
-        MultiPoint(points, bbox)
+        MultiPoint(points, bbox, foreignMembers)
 
     operator fun Position.unaryPlus() {
         points.add(this)
@@ -63,9 +71,9 @@ inline fun multiPoint(block: MultiPointDsl.() -> Unit): MultiPoint = MultiPointD
     .apply(block).create()
 
 @GeoJsonDsl
-class LineStringDsl(internal val points: MutableList<Position> = mutableListOf()) : GeometryDsl<LineString>() {
+class LineStringDsl(private val points: MutableList<Position> = mutableListOf()) : GeometryDsl<LineString>() {
     override fun create(): LineString =
-        LineString(points, bbox)
+        LineString(points, bbox, foreignMembers)
 
     operator fun Position.unaryPlus() {
         points.add(this)
@@ -88,7 +96,7 @@ inline fun lineString(block: LineStringDsl.() -> Unit) = LineStringDsl()
 class MultiLineStringDsl(private val coordinates: MutableList<List<Position>> = mutableListOf()) :
     GeometryDsl<MultiLineString>() {
     override fun create(): MultiLineString =
-        MultiLineString(coordinates)
+        MultiLineString(coordinates, bbox, foreignMembers)
 
     inline fun lineString(block: LineStringDsl.() -> Unit) {
         +LineStringDsl().apply(block).create()
@@ -106,7 +114,7 @@ inline fun multiLineString(block: MultiLineStringDsl.() -> Unit) = MultiLineStri
 @GeoJsonDsl
 class PolygonDsl(internal val coordinates: MutableList<List<Position>> = mutableListOf()) : GeometryDsl<Polygon>() {
     override fun create(): Polygon =
-        Polygon(coordinates, bbox)
+        Polygon(coordinates, bbox, foreignMembers)
 
     inner class RingDsl(internal val points: MutableList<Position> = mutableListOf()) {
         operator fun Position.unaryPlus() {
@@ -147,7 +155,7 @@ inline fun polygon(block: PolygonDsl.() -> Unit) = PolygonDsl()
 class MultiPolygonDsl(private val coordinates: MutableList<List<List<Position>>> = mutableListOf()) :
     GeometryDsl<MultiPolygon>() {
     override fun create(): MultiPolygon =
-        MultiPolygon(coordinates, bbox)
+        MultiPolygon(coordinates, bbox, foreignMembers)
 
     inline fun polygon(block: PolygonDsl.() -> Unit) {
         +PolygonDsl().apply(block).create()
@@ -167,7 +175,7 @@ inline fun multiPolygon(block: MultiPolygonDsl.() -> Unit) = MultiPolygonDsl()
 class GeometryCollectionDsl(private val geometries: MutableList<Geometry> = mutableListOf()) :
     GeometryDsl<GeometryCollection>() {
     override fun create(): GeometryCollection =
-        GeometryCollection(geometries)
+        GeometryCollection(geometries, bbox, foreignMembers)
 
     operator fun Geometry.unaryPlus() {
         geometries.add(this)
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureCollectionSerializer.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureCollectionSerializer.kt
index ed7bb5d9..b56d7329 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureCollectionSerializer.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureCollectionSerializer.kt
@@ -29,6 +29,7 @@ object FeatureCollectionSerializer : JsonSerializer<FeatureCollection> {
                     value.features.forEach { add(it.toJsonObject()) }
                 }
             )
+            value.foreignMembers.forEach { (key, value) -> put(key, value) }
         }
         output.encodeJsonElement(data)
     }
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureSerializer.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureSerializer.kt
index 8c3a5ba1..b3ba1a10 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureSerializer.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureSerializer.kt
@@ -28,5 +28,7 @@ object FeatureSerializer : JsonSerializer<Feature> {
         id?.let { put("id", it) }
 
         put("properties", JsonObject(properties))
+
+        foreignMembers.forEach { (key, value) -> put(key, value) }
     }
 }
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/GeometrySerializer.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/GeometrySerializer.kt
index ea86c351..81c1d66c 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/GeometrySerializer.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/GeometrySerializer.kt
@@ -125,6 +125,7 @@ object GeometrySerializer : KSerializer<Geometry> {
         }
 
         bbox?.let { put("bbox", it.toJsonArray()) }
+        foreignMembers.forEach { (key, value) -> put(key, value) }
     }
 
 
diff --git a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/Utils.kt b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/Utils.kt
index 51218f0f..032c46f2 100644
--- a/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/Utils.kt
+++ b/geojson/src/commonMain/kotlin/io/github/dellisd/spatialk/geojson/serialization/Utils.kt
@@ -2,8 +2,12 @@ package io.github.dellisd.spatialk.geojson.serialization
 
 import io.github.dellisd.spatialk.geojson.BoundingBox
 import io.github.dellisd.spatialk.geojson.Feature
+import io.github.dellisd.spatialk.geojson.GeoJson
 import io.github.dellisd.spatialk.geojson.Position
+import kotlinx.serialization.json.Json
 import kotlinx.serialization.json.JsonArray
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.buildJsonArray
 import kotlinx.serialization.json.double
 import kotlinx.serialization.json.jsonArray
@@ -19,8 +23,31 @@ internal fun BoundingBox?.jsonProp(): String = if (this == null) "" else """"bbo
 
 internal fun Feature.idProp(): String = if (this.id == null) "" else """"id":"${this.id}","""
 
+internal fun GeoJson.foreignMembersJsonProps(): String {
+    if (this.foreignMembers.isEmpty()) return ""
+    return this.foreignMembers.entries.joinToString(prefix = ",", separator = ",") {
+        """"${it.key}":${Json.encodeToString(JsonElement.serializer(), it.value)}"""
+    }
+}
+
 internal fun JsonArray.toPosition(): Position =
     Position(this[0].jsonPrimitive.double, this[1].jsonPrimitive.double, this.getOrNull(2)?.jsonPrimitive?.double)
 
 internal fun JsonArray.toBbox(): BoundingBox =
     BoundingBox(this.map { it.jsonPrimitive.double }.toDoubleArray())
+
+private val FEATURE_MEMBERS = listOf("type", "geometry", "properties", "id", "bbox")
+private val FEATURE_COLLECTION_MEMBERS = listOf("type", "features", "bbox")
+private val GEOMETRY_COLLECTION_MEMBERS = listOf("type", "geometries", "bbox")
+private val GEOMETRY_ELEMENT_MEMBERS = listOf("type", "coordinates", "bbox")
+
+internal fun JsonObject.foreignMembers(): Map<String, JsonElement> {
+    val knownMembers = when(this.getValue("type").jsonPrimitive.content) {
+        "Feature"            -> FEATURE_MEMBERS
+        "FeatureCollection"  -> FEATURE_COLLECTION_MEMBERS
+        "GeometryCollection" -> GEOMETRY_COLLECTION_MEMBERS
+        else                 -> GEOMETRY_ELEMENT_MEMBERS
+    }
+    return this.filterKeys { !knownMembers.contains(it) }
+}
+
diff --git a/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/dsl/GeoJsonDslTests.kt b/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/dsl/GeoJsonDslTests.kt
index 4a2164f2..aacce879 100644
--- a/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/dsl/GeoJsonDslTests.kt
+++ b/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/dsl/GeoJsonDslTests.kt
@@ -5,6 +5,9 @@ import io.github.dellisd.spatialk.geojson.LineString
 import io.github.dellisd.spatialk.geojson.MultiPoint
 import io.github.dellisd.spatialk.geojson.Polygon
 import io.github.dellisd.spatialk.geojson.Position
+import kotlinx.serialization.json.JsonArray
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.JsonPrimitive
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertFails
@@ -77,7 +80,22 @@ class GeoJsonDslTests {
             +simplePoint
             +simpleLine
             +simplePolygon
+            +point(-70.0, 40.0) {
+                foreignMembers {
+                    put("geometry extension", "value")
+                }
+            }
+        }, foreignMembers = {
+            put("fm string", "str")
+            put("fm number", 5)
+            put("fm array", JsonArray(listOf(JsonPrimitive("elem1"), JsonPrimitive("elem2"))))
+            put("fm bool", true)
+            put("fm object", JsonObject(mapOf("prop1" to JsonPrimitive("value1"), "prop2" to JsonPrimitive("value2"))))
         })
+
+        foreignMembers {
+            put("feature collection extension", "value")
+        }
     }
 
     private val collectionJson =
@@ -92,7 +110,10 @@ class GeoJsonDslTests {
 |[3.0,3.0],[4.0,4.0]]],[[[12.0,0.0],[0.0,12.0],[-12.0,0.0],[5.0,5.0],[12.0,0.0]]]]},"properties":{}},{"type":"Feature",
 |"geometry":{"type":"GeometryCollection","geometries":[{"type":"Point","coordinates":[-75.0,45.0,100.0]},
 |{"type":"LineString","coordinates":[[45.0,45.0],[0.0,0.0]]},{"type":"Polygon","coordinates":[[[45.0,45.0],[0.0,0.0],
-|[12.0,12.0],[45.0,45.0]],[[4.0,4.0],[2.0,2.0],[3.0,3.0],[4.0,4.0]]]}]},"properties":{}}]}""".trimMargin()
+|[12.0,12.0],[45.0,45.0]],[[4.0,4.0],[2.0,2.0],[3.0,3.0],[4.0,4.0]]]},{"type":"Point","coordinates":[-70.0,40.0],"geometry extension":"value"}]},
+|"properties":{},"fm string":"str","fm number":5,"fm array":["elem1","elem2"],"fm bool":true,"fm object":{"prop1":"value1","prop2":"value2"}}],
+|"feature collection extension":"value"}
+|""".trimMargin()
 
     @Test
     fun testDslConstruction() {
diff --git a/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureCollectionSerializationTests.kt b/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureCollectionSerializationTests.kt
index a2624ca4..131844a6 100644
--- a/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureCollectionSerializationTests.kt
+++ b/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureCollectionSerializationTests.kt
@@ -22,12 +22,15 @@ class FeatureCollectionSerializationTests {
                 "name" to JsonPrimitive("Nowhere")
             )
         )
-        val collection = FeatureCollection(feature, feature)
+        val collection = FeatureCollection(features = listOf(feature, feature), foreignMembers = mapOf(
+            "ext" to JsonPrimitive("extension value")
+        ))
 
         val json =
-            """{"type":"FeatureCollection","features":[{"type":"Feature","geometry":{"type":"Point","coordinates":
-                |[12.3,45.6]},"properties":{"size":45.1,"name":"Nowhere"}},{"type":"Feature","geometry":{"type":"Point",
-                |"coordinates":[12.3,45.6]},"properties":{"size":45.1,"name":"Nowhere"}}]}"""
+            """{"type":"FeatureCollection","features":[
+                |{"type":"Feature","geometry":{"type":"Point","coordinates":[12.3,45.6]},"properties":{"size":45.1,"name":"Nowhere"}},
+                |{"type":"Feature","geometry":{"type":"Point","coordinates":[12.3,45.6]},"properties":{"size":45.1,"name":"Nowhere"}}],
+                |"ext":"extension value"}"""
                 .trimMargin()
                 .replace("\n", "")
 
@@ -44,14 +47,17 @@ class FeatureCollectionSerializationTests {
                 "name" to JsonPrimitive("Nowhere")
             )
         )
-        val collection = FeatureCollection(feature, feature)
+        val collection = FeatureCollection(features = listOf(feature, feature), foreignMembers = mapOf(
+            "ext" to JsonPrimitive("extension value")
+        ))
 
         assertEquals(
             collection,
             FeatureCollection.fromJson(
-                """{"type":"FeatureCollection","features":[{"type":"Feature","geometry":{"type":"Point","coordinates":
-                |[12.3,45.6]},"properties":{"size":45.1,"name":"Nowhere"}},{"type":"Feature","geometry":{"type":"Point",
-                |"coordinates":[12.3,45.6]},"properties":{"size":45.1,"name":"Nowhere"}}]}"""
+                """{"type":"FeatureCollection","features":[
+                    |{"type":"Feature","geometry":{"type":"Point","coordinates":[12.3,45.6]},"properties":{"size":45.1,"name":"Nowhere"}},
+                    |{"type":"Feature","geometry":{"type":"Point","coordinates":[12.3,45.6]},"properties":{"size":45.1,"name":"Nowhere"}}],
+                    |"ext":"extension value"}"""
                     .trimMargin()
                     .replace("\n", "")
             )
diff --git a/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureSerializationTests.kt b/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureSerializationTests.kt
index e302f6ca..a58f0401 100644
--- a/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureSerializationTests.kt
+++ b/geojson/src/commonTest/kotlin/io/github/dellisd/spatialk/geojson/serialization/FeatureSerializationTests.kt
@@ -15,7 +15,9 @@ class FeatureSerializationTests {
 
     @Test
     fun testSerializeFeature() {
-        val geometry = Point(Position(12.3, 45.6))
+        val geometry = Point(Position(12.3, 45.6), foreignMembers = mapOf(
+            "geom ext" to JsonPrimitive("extension value")
+        ))
         val feature = Feature(
             geometry,
             mapOf(
@@ -23,12 +25,19 @@ class FeatureSerializationTests {
                 "name" to JsonPrimitive("Nowhere")
             ),
             "001",
-            BoundingBox(11.6, 45.1, 12.7, 45.7)
+            BoundingBox(11.6, 45.1, 12.7, 45.7),
+            foreignMembers = mapOf(
+                "ext" to JsonPrimitive("extension value")
+            )
         )
 
         val json =
-            """{"type":"Feature","bbox":[11.6,45.1,12.7,45.7],"geometry":{"type":"Point","coordinates":[12.3,45.6]},
-                |"id":"001","properties":{"size":45.1,"name":"Nowhere"}}
+            """{"type":"Feature",
+                |"bbox":[11.6,45.1,12.7,45.7],
+                |"geometry":{"type":"Point","coordinates":[12.3,45.6],"geom ext":"extension value"},
+                |"id":"001",
+                |"properties":{"size":45.1,"name":"Nowhere"},
+                |"ext":"extension value"}
             """.trimMargin().replace("\n", "")
 
         assertEquals(json, feature.json(), "Feature (fast)")
@@ -37,7 +46,9 @@ class FeatureSerializationTests {
 
     @Test
     fun testDeserializeFeature() {
-        val geometry = Point(Position(12.3, 45.6))
+        val geometry = Point(Position(12.3, 45.6), foreignMembers = mapOf(
+            "geom ext" to JsonPrimitive("extension value")
+        ))
         val feature = Feature(
             geometry,
             properties = mapOf(
@@ -45,7 +56,10 @@ class FeatureSerializationTests {
                 "name" to JsonPrimitive("Nowhere")
             ),
             id = "001",
-            bbox = BoundingBox(11.6, 45.1, 12.7, 45.7)
+            bbox = BoundingBox(11.6, 45.1, 12.7, 45.7),
+            foreignMembers = mapOf(
+                "ext" to JsonPrimitive("extension value")
+            )
         )
 
         assertEquals(
@@ -55,12 +69,14 @@ class FeatureSerializationTests {
                 |"bbox":[11.6,45.1,12.7,45.7],
                 |"geometry":{
                     |"type":"Point",
-                    |"coordinates":[12.3,45.6]},
+                    |"coordinates":[12.3,45.6],
+                    |"geom ext":"extension value"},
                 |"id":"001",
                 |"properties":{
                     |"size":45.1,
                     |"name":"Nowhere"
-                |}}
+                |},
+                |"ext":"extension value"}
             """.trimMargin().replace("\n", "")
             )
         )