diff --git a/app/controllers/sites_controller.rb b/app/controllers/sites_controller.rb
index 67385105a..a8f64d1e1 100644
--- a/app/controllers/sites_controller.rb
+++ b/app/controllers/sites_controller.rb
@@ -1,11 +1,27 @@
 class SitesController < ApplicationController
-  before_action :find_site
+  before_action :find_site, only: %i[edit update show]
+  before_action :find_organisation, only: %i[new create]
   before_action :check_user_is_gds_editor, only: %i[edit update]
+  before_action :check_user_is_site_manager, only: %i[new create]
+
+  def new
+    @site_form = SiteForm.new(organisation_slug: @organisation.whitehall_slug)
+  end
+
+  def create
+    @site_form = SiteForm.new(create_params)
+
+    if (site = @site_form.save)
+      redirect_to site_path(site), flash: { success: "Transition site created" }
+    else
+      render :new
+    end
+  end
 
   def edit; end
 
   def update
-    if @site.update(site_params)
+    if @site.update(update_params)
       redirect_to site_path(@site), flash: { success: "Transition date updated" }
     else
       redirect_to edit_site_path(@site), flash: { alert: "We couldn't save your change" }
@@ -24,7 +40,30 @@ def find_site
     @site = Site.find_by!(abbr: params[:id])
   end
 
-  def site_params
+  def find_organisation
+    @organisation = Organisation.find_by(whitehall_slug: params[:organisation_id])
+  end
+
+  def create_params
+    params.require(:site_form).permit(
+      :organisation_slug,
+      :abbr,
+      :tna_timestamp,
+      :homepage,
+      :homepage_title,
+      :global_type,
+      :global_new_url,
+      :global_redirect_append_path,
+      :homepage_furl,
+      :hostname,
+      :query_params,
+      :special_redirect_strategy,
+      :aliases,
+      extra_organisations: [],
+    )
+  end
+
+  def update_params
     params.require(:site).permit(:launch_date)
   end
 
@@ -34,4 +73,11 @@ def check_user_is_gds_editor
       redirect_to site_path(@site), alert: message
     end
   end
+
+  def check_user_is_site_manager
+    unless current_user.site_manager?
+      message = "Only Site Managers can access that."
+      redirect_to organisation_path(@organisation), alert: message
+    end
+  end
 end
diff --git a/app/forms/site_form.rb b/app/forms/site_form.rb
new file mode 100644
index 000000000..3797dad1d
--- /dev/null
+++ b/app/forms/site_form.rb
@@ -0,0 +1,105 @@
+class SiteForm
+  include ActiveModel::Model
+  include ActiveModel::Attributes
+
+  attribute :organisation_slug
+  attribute :abbr
+  attribute :tna_timestamp, :datetime
+  attribute :homepage
+  attribute :homepage_title
+  attribute :extra_organisations
+  attribute :global_type
+  attribute :global_new_url
+  attribute :homepage_furl
+  attribute :query_params
+  attribute :global_redirect_append_path, :boolean, default: false
+  attribute :special_redirect_strategy
+
+  attribute :hostname
+  attribute :aliases
+
+  validate :validate_children
+  validate :validate_aliases
+  validate :aliases_are_unique
+
+  def save
+    return false if invalid?
+
+    ActiveRecord::Base.transaction do
+      site.save!
+      hosts.each(&:save!)
+      aka_hosts.each(&:save!)
+    end
+
+    site
+  end
+
+  def organisations
+    Organisation.where.not(whitehall_slug: organisation_slug)
+  end
+
+private
+
+  def site
+    @site ||= Site.new(
+      abbr:,
+      tna_timestamp:,
+      homepage:,
+      organisation: Organisation.find_by(whitehall_slug: organisation_slug),
+      extra_organisations: Organisation.where(title: extra_organisations),
+      homepage_title:,
+      homepage_furl:,
+      global_type:,
+      global_new_url:,
+      global_redirect_append_path:,
+      query_params:,
+      special_redirect_strategy:,
+    )
+  end
+
+  def hosts
+    [default_host].concat(alias_hosts)
+  end
+
+  def aka_hosts
+    hosts.map { |host| build_aka_host(host) }
+  end
+
+  def default_host
+    @default_host ||= Host.new(hostname:, site:)
+  end
+
+  def alias_hosts
+    return [] if aliases.nil?
+
+    @alias_hosts ||= aliases.split(",").map do |host|
+      Host.new(hostname: host, site:)
+    end
+  end
+
+  def build_aka_host(host)
+    Host.new(hostname: host.aka_hostname, canonical_host: host, site:)
+  end
+
+  def validate_children
+    [site, default_host].each do |child|
+      errors.merge!(child.errors) if child.invalid?
+    end
+  end
+
+  def validate_aliases
+    alias_hosts.each do |host|
+      next if host.valid?
+
+      host.errors.each do |error|
+        errors.add(:aliases, "\"#{host.hostname}\" #{error.message}")
+      end
+    end
+  end
+
+  def aliases_are_unique
+    if alias_hosts.length != alias_hosts.map(&:hostname).uniq.length
+      errors.add(:aliases, "must be unique")
+    end
+  end
+end
diff --git a/app/models/concerns/nilify_blanks.rb b/app/models/concerns/nilify_blanks.rb
index 100c2a725..2630f3b42 100644
--- a/app/models/concerns/nilify_blanks.rb
+++ b/app/models/concerns/nilify_blanks.rb
@@ -7,7 +7,15 @@ module NilifyBlanks
 
   def nilify_blanks
     attributes.each do |column, value|
+      next if nilify_except.include?(column.to_sym)
+
       self[column] = nil if value.blank?
     end
   end
+
+private
+
+  def nilify_except
+    []
+  end
 end
diff --git a/app/models/host.rb b/app/models/host.rb
index 7e93a51ef..35cd1fcd1 100644
--- a/app/models/host.rb
+++ b/app/models/host.rb
@@ -11,7 +11,8 @@ class Host < ApplicationRecord
   validates :hostname, presence: true
   validates :hostname, hostname: true
   validates :site, presence: true
-  validate :canonical_host_id_xor_aka_present
+  validate :canonical_host_id_xor_aka_present, if: -> { hostname.present? }
+  validate :hostname_is_downcased, if: -> { hostname.present? }
 
   after_update :update_hits_relations, if: :saved_change_to_site_id?
 
@@ -71,4 +72,10 @@ def update_hits_relations
     hits.update_all(mapping_id: nil)
     Transition::Import::HitsMappingsRelations.refresh!(site)
   end
+
+  def hostname_is_downcased
+    if hostname != hostname.downcase
+      errors.add(:hostname, "must be lowercase")
+    end
+  end
 end
diff --git a/app/models/site.rb b/app/models/site.rb
index 2fa229459..0b9edcf2d 100644
--- a/app/models/site.rb
+++ b/app/models/site.rb
@@ -2,6 +2,11 @@
 require "./lib/transition/path_or_url"
 
 class Site < ApplicationRecord
+  include NilifyBlanks
+
+  GLOBAL_TYPES = %w[redirect archive].freeze
+  SPECIAL_REDIRECT_STRATEGY_TYPES = %w[via_aka supplier].freeze
+
   belongs_to :organisation
 
   has_many :hosts
@@ -20,8 +25,9 @@ class Site < ApplicationRecord
   validates :organisation, presence: true
   validates :homepage, presence: true, non_blank_url: true
   validates :abbr, uniqueness: true, presence: true, format: { with: /\A[a-zA-Z0-9_-]+\z/, message: "can only contain alphanumeric characters, underscores and dashes" }
-  validates :special_redirect_strategy, inclusion: { in: %w[via_aka supplier], allow_nil: true }
+  validates :special_redirect_strategy, inclusion: { in: SPECIAL_REDIRECT_STRATEGY_TYPES, allow_blank: true }
   validates :global_new_url, presence: { if: :global_redirect? }
+  validates :global_new_url, absence: { if: :global_archive? }
   validates :global_new_url,
             format: { without: /\?/,
                       message: "cannot contain a query when the path is appended",
@@ -131,4 +137,8 @@ def should_remove_unused_view?
   def remove_all_hits_view
     Postgres::MaterializedView.drop(precomputed_view_name)
   end
+
+  def nilify_except
+    %i[global_redirect_append_path query_params precompute_all_hits_view]
+  end
 end
diff --git a/app/views/organisations/show.html.erb b/app/views/organisations/show.html.erb
index f566e3c51..ef1d77b46 100644
--- a/app/views/organisations/show.html.erb
+++ b/app/views/organisations/show.html.erb
@@ -10,6 +10,10 @@
   <%= render 'in_conjunction_with' %>
 </header>
 
+<% if current_user.site_manager? %>
+  <%= link_to "Add a transition site", new_organisation_site_path(@organisation), class: "btn btn-default" %>
+<% end %>
+
 <% unless @sites.empty? %>
   <h2>Sites</h2>
   <table class="sites table table-hover table-striped table-bordered" data-module="filterable-table">
diff --git a/app/views/sites/new.html.erb b/app/views/sites/new.html.erb
new file mode 100644
index 000000000..98bb83d17
--- /dev/null
+++ b/app/views/sites/new.html.erb
@@ -0,0 +1,120 @@
+<% breadcrumb(:new_site, @organisation) %>
+
+<div class="page-title-with-border">
+  <h1>New transition site</h1>
+</div>
+
+<%= form_for @site_form, html: { role: "form" }, url: organisation_sites_path do |form| %>
+  <%= render "shared/error_messages", error_messages: form.object.errors.full_messages %>
+
+  <div class="form-group row">
+    <div class="col-md-8">
+      <%= form.hidden_field :organisation_slug, value: @organisation.whitehall_slug %>
+
+      <div class="form-group">
+        <%= form.label :abbr %>
+        <%= form.text_field :abbr, class: "form-control", aria: { describedby: "abbr-help" } %>
+        <small id="abbr-help" class="form-text text-muted"><%= I18n.t("activemodel.hint.site_form.abbr") %></small>
+      </div>
+
+      <div class="form-group">
+        <%= form.label :tna_timestamp %>
+        <%= form.text_field :tna_timestamp, class: "form-control", aria: { describedby: "tna-timestamp-help" } %>
+        <small id="tna-timestamp-help" class="form-text text-muted"><%= I18n.t("activemodel.hint.site_form.tna_timestamp").html_safe %></small>
+      </div>
+
+      <div class="form-group">
+        <%= form.label :homepage %>
+        <%= form.text_field :homepage, class: "form-control", aria: { describedby: "homepage-help" } %>
+        <small id="homepage-help" class="form-text text-muted"><%= I18n.t("activemodel.hint.site_form.homepage") %></small>
+      </div>
+
+      <div class="form-group">
+        <%= form.label :hostname %>
+        <%= form.text_field :hostname, class: "form-control", aria: { describedby: "hostname-help" } %>
+        <small id="hostname-help" class="form-text text-muted"><%= I18n.t("activemodel.hint.site_form.hostname") %></small>
+      </div>
+
+      <div class="form-group">
+        <%= form.label :homepage_title %>
+        <%= form.text_field :homepage_title, class: "form-control", aria: { describedby: "homepage-title-help" } %>
+        <small id="homepage-title-help" class="form-text text-muted"><%= I18n.t("activemodel.hint.site_form.homepage_title") %></small>
+      </div>
+
+      <div class="form-group">
+        <%= form.label :extra_organisations %>
+        <%= form.collection_select :extra_organisations,
+                                   form.object.organisations,
+                                   :id,
+                                   :title,
+                                   {},
+                                   { multiple: true, class: "form-control", aria: { describedby: "extra-organisations-help" } }
+        %>
+        <small id="extra-organisations-help" class="form-text text-muted"><%= I18n.t("activemodel.hint.site_form.extra_organisations") %></small>
+      </div>
+
+      <div class="form-group">
+        <%= form.label :homepage_furl %>
+        <%= form.text_field :homepage_furl, class: "form-control", aria: { describedby: "homepage-furl-help" } %>
+        <small id="homepage-furl-help" class="form-text text-muted"><%= I18n.t("activemodel.hint.site_form.homepage_furl") %></small>
+      </div>
+
+      <div class="form-group">
+        <%= form.label :global_type %>
+        <%= form.collection_radio_buttons :global_type,
+                                          Site::GLOBAL_TYPES,
+                                          :to_s,
+                                          :humanize,
+                                          aria: { describedby: "global-type-help" } do |button| %>
+          <div class="radio">
+            <%= button.label do %>
+              <%= button.radio_button + button.text %>
+            <% end %>
+          </div>
+        <% end %>
+        <small id="global-type-help" class="form-text text-muted"><%= I18n.t("activemodel.hint.site_form.global_type") %></small>
+      </div>
+
+      <div class="form-group">
+        <%= form.label :global_new_url %>
+        <%= form.text_field :global_new_url, class: "form-control" %>
+      </div>
+
+      <div class="form-group">
+        <%= form.label :query_params %>
+        <%= form.text_field :query_params, class: "form-control", aria: { describedby: "query-params-help" } %>
+        <small id="query-params-help" class="form-text text-muted row"><%= I18n.t("activemodel.hint.site_form.query_params") %></small>
+      </div>
+
+      <div class="form-check">
+        <%= form.label :global_redirect_append_path %>
+        <%= form.check_box :global_redirect_append_path, aria: { describedby: "global-redirect-append-path-help" } %>
+        <small id="global-redirect-append-path-help" class="form-text text-muted row"><%= I18n.t("activemodel.hint.site_form.global_redirect_append_path") %></small>
+      </div>
+
+      <div class="form-group">
+        <%= form.label :special_redirect_strategy %>
+        <%= form.collection_radio_buttons :special_redirect_strategy,
+                                          Site::SPECIAL_REDIRECT_STRATEGY_TYPES,
+                                          :to_s,
+                                          :humanize,
+                                          aria: { describedby: "special-redirect-strategy-help" } do |button| %>
+          <div class="radio">
+            <%= button.label do %>
+              <%= button.radio_button + button.text %>
+            <% end %>
+          </div>
+        <% end %>
+        <small id="special-redirect-strategy-help" class="form-text text-muted"><%= I18n.t("activemodel.hint.site_form.special_redirect_strategy") %></small>
+      </div>
+
+      <div class="form-group">
+        <%= form.label :aliases %>
+        <%= form.text_area :aliases, class: "form-control", aria: { describedby: "aliases-help" } %>
+        <small id="aliases-help" class="form-text text-muted"><%= I18n.t("activemodel.hint.site_form.aliases") %></small>
+      </div>
+
+      <%= form.submit 'Save', class: 'add-top-margin btn btn-success' %>
+    </div>
+  </div>
+<% end %>
diff --git a/config/breadcrumbs.rb b/config/breadcrumbs.rb
index 87824111c..64a987f77 100644
--- a/config/breadcrumbs.rb
+++ b/config/breadcrumbs.rb
@@ -7,6 +7,11 @@
   parent :root
 end
 
+crumb :new_site do |organisation|
+  link "New transition site"
+  parent :organisation, organisation
+end
+
 crumb :site do |site|
   link site.default_host.hostname, site_path(site)
   parent :organisation, site.organisation
diff --git a/config/locales/en.yml b/config/locales/en.yml
index fe6ef44e6..4c2551e0c 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -16,6 +16,38 @@ en:
         new_url: 'The URL to redirect to'
       mappings_batch:
         paths: Old URLs
+  activemodel:
+    attributes:
+      site_form:
+        abbr: Abbreviated name
+        host: Host
+        tna_timestamp: TNA timestamp
+        homepage: Homepage
+        homepage_title: Homepage title (Optional)
+        global_type: Global type (Optional)
+        global_new_url: Global new URL (Optional)
+        extra_organisations: Extra organisations (Optional)
+        homepage_furl: Homepage full URL (Optional)
+        options: Options (Optional)
+        global_redirect_append_path: Global redirect append path (Optional)
+        query_params: Query params (Optional)
+        special_redirect_strategy: Special redirect strategy (Optional)
+        aliases: Aliases (Optional)
+    hint:
+      site_form:
+        abbr: This is the owning organisation abbreviation followed by the abbreviated site name, separated by an underscore. For example, the "Obesity West Midlands" site which is owned by Public Health England would be "phe_obesitywm"
+        host: This is the primary hostname for the site. For example, "www.example.com".
+        tna_timestamp: This is the last good capture from the <a href="https://www.nationalarchives.gov.uk/webarchive/" class="govuk-link">UK Government Web Archives</a>. For example, "20131002172858". If the site has not been crawled by the National Archives, set a stub timestamp "20201010101010".
+        homepage: This is the URL for the new site.
+        hostname: This is the primary hostname for the site.
+        homepage_title: This is the title for 404/410 pages, it defaults to the organisation title. It should fit into the sentence "Visit the new [title] site at [full URL or homepage]"
+        extra_organisations: These are the additional organisations which own this site. They are used for access control in Transition.
+        homepage_furl: This is the friendly URL displayed on 404/410 pages. It should redirect to the homepage. It doesn't need to include 'http' or 'https'.
+        global_type: This sets a global redirect or archive for all paths. If "Redirect", all site URLs will redirect to the Global new URL. If "Archive", all site URLs will show a page saying the site has been archived.
+        query_params:  A significant querystring parameter is one which on the old website changes the content in a meaningful way - which we might therefore need to map to a different place. Query string parameters should be specified in lowercase; uppercase parameters will not be preserved during canonicalisation. Enter as a colon-separated list.
+        global_redirect_append_path: Should the path the user supplied be appended to the URL for the global redirect?
+        special_redirect_strategy: When the transition is partial, some tools or content will be left behind and managed by the previous supplier. If "Via aka", the supplier is redirecting some paths to our aka domain. If "supplier", the supplier is managing redirects to gov.uk. No traffic comes through Bouncer for this site.
+        aliases: This is a list of alias domains. Enter as a comma-separated list.
   mappings:
     success:
       all_created: '%{created}%{tagged_with}'
diff --git a/config/routes.rb b/config/routes.rb
index fa5ba4951..62b348eb5 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -19,7 +19,9 @@
 
   resources :hosts, only: [:index]
 
-  resources :organisations, only: %i[show index]
+  resources :organisations, only: %i[show index] do
+    resources :sites, only: %i[new create], controller: :sites
+  end
 
   get "mappings/find_global", to: "mappings#find_global"
   get "hits", to: "hits#universal_summary"
diff --git a/features/site.feature b/features/site.feature
index 485257052..f47bf2d07 100644
--- a/features/site.feature
+++ b/features/site.feature
@@ -102,6 +102,19 @@ Scenario: Jumping to a non-existent site
   And I jump to the site or mapping "http://not-a-site.gov.uk"
   Then I should see the header "Unknown site"
 
+Scenario: Creating a site
+  Given I have logged in as a Site Manager
+  And there are these organisations without sites:
+    | whitehall_slug  | title                         |
+    | ukti            | UK Trade & Industry           |
+    | go-science      | Government Office for Science |
+  When I visit the page for the UK Trade & Industry organisation
+  And I click the link "Add a transition site"
+  Then I should be on the new transition site page for the UK Trade & Industry organisation
+  When I fill in the new transition site fields
+  And I save my changes
+  Then I should be redirected to the new site
+
 Scenario: Editing a site's transition date as a GDS Editor
   Given I have logged in as a GDS Editor
   And the date is 29/11/19
diff --git a/features/step_definitions/site_interaction_steps.rb b/features/step_definitions/site_interaction_steps.rb
index 5c3e09bd7..020b07645 100644
--- a/features/step_definitions/site_interaction_steps.rb
+++ b/features/step_definitions/site_interaction_steps.rb
@@ -1,3 +1,8 @@
+When(/^I visit the page for the (.*) organisation$/) do |organisation_title|
+  organisation = Organisation.find_by(title: organisation_title)
+  visit organisation_path(organisation)
+end
+
 When(/^I visit this site page$/) do
   visit site_path(@site)
 end
@@ -9,3 +14,28 @@
   select("20", from: "site_launch_date_3i")
   click_button "Save"
 end
+
+When(/^I fill in the new transition site fields/) do
+  fill_in "Abbreviated name", with: "aaib"
+  fill_in "TNA timestamp", with: "20141104112824"
+  fill_in "Homepage", with: "https://www.gov.uk/government/organisations/air-accidents-investigation-branch"
+  fill_in "Hostname", with: "www.aaib.gov.uk"
+  fill_in "Homepage title", with: "Air accidents investigation branch"
+  select "Government Office for Science"
+  fill_in "Homepage full URL", with: "www.gov.uk/aaib"
+  choose "Redirect"
+  fill_in "Global new URL", with: "https://www.gov.uk/government/organisations/air-accidents-investigation-branch/about"
+  fill_in "Query params", with: "file"
+  check "Global redirect append path"
+  choose "Via aka"
+  fill_in "Aliases", with: "aaib.gov.uk,aaib.com"
+end
+
+Then(/^I should be on the new transition site page for the (.*) organisation$/) do |organisation_title|
+  organisation = Organisation.find_by(title: organisation_title)
+  i_should_be_on_the_path new_organisation_site_path(organisation)
+end
+
+Then(/^I should be redirected to the new site$/) do
+  i_should_be_on_the_path site_path(Site.last)
+end
diff --git a/spec/controllers/sites_controller_spec.rb b/spec/controllers/sites_controller_spec.rb
index 91731622e..85e56957b 100644
--- a/spec/controllers/sites_controller_spec.rb
+++ b/spec/controllers/sites_controller_spec.rb
@@ -3,6 +3,48 @@
 describe SitesController do
   let(:site)    { create :site, abbr: "moj" }
   let(:gds_bob) { create(:gds_editor, name: "Bob Terwhilliger") }
+  let(:site_manager) { create(:site_manager) }
+
+  describe "#new" do
+    let(:organisation) { create(:organisation) }
+
+    before { login_as site_manager }
+
+    it "returns a success response" do
+      get :new, params: { organisation_id: organisation.whitehall_slug }
+
+      expect(response.status).to eql(200)
+    end
+
+    context "when the user does not have permission" do
+      def make_request
+        get :new, params: { organisation_id: organisation.whitehall_slug }
+      end
+
+      it_behaves_like "disallows editing by non-Site managers"
+    end
+  end
+
+  describe "#create" do
+    let(:organisation) { create(:organisation) }
+    let(:params) { attributes_for(:site_form) }
+
+    before { login_as site_manager }
+
+    it "returns a success response" do
+      post :create, params: { organisation_id: organisation.whitehall_slug, site_form: params }
+
+      expect(response.status).to eql(200)
+    end
+
+    context "when the user does not have permission" do
+      def make_request
+        post :create, params: { organisation_id: organisation.whitehall_slug, site_form: params }
+      end
+
+      it_behaves_like "disallows editing by non-Site managers"
+    end
+  end
 
   describe "#edit" do
     context "when the user does have permission" do
diff --git a/spec/factories/site_forms.rb b/spec/factories/site_forms.rb
new file mode 100644
index 000000000..433cf23e7
--- /dev/null
+++ b/spec/factories/site_forms.rb
@@ -0,0 +1,37 @@
+FactoryBot.define do
+  factory :site_form do
+    abbr { "aaib" }
+    organisation_slug { "air-accidents-investigation-branch" }
+    homepage { "https://www.gov.uk/government/organisations/air-accidents-investigation-branch" }
+    tna_timestamp { "20141104112824" }
+    hostname { "www.aaib.gov.uk" }
+
+    trait :with_optional_fields do
+      homepage_title { "Air accidents investigation branch" }
+      homepage_furl { "www.gov.uk/aaib" }
+      global_type { "redirect" }
+      global_new_url { "https://www.gov.uk/government/organisations/air-accidents-investigation-branch/about" }
+      query_params { "file" }
+      global_redirect_append_path { true }
+      special_redirect_strategy { "via_aka" }
+    end
+
+    trait :with_blank_optional_fields do
+      homepage_title { "" }
+      homepage_furl { "" }
+      global_type { "" }
+      global_new_url { "" }
+      query_params { "" }
+      global_redirect_append_path { false }
+      special_redirect_strategy { "" }
+    end
+
+    trait :with_aliases do
+      aliases { "aaib.gov.uk,aaib.com" }
+    end
+
+    trait :with_extra_organisations do
+      extra_organisations { ["The adjudicator's office", "Government digital service"] }
+    end
+  end
+end
diff --git a/spec/forms/site_form_spec.rb b/spec/forms/site_form_spec.rb
new file mode 100644
index 000000000..9b6cf88dd
--- /dev/null
+++ b/spec/forms/site_form_spec.rb
@@ -0,0 +1,139 @@
+require "rails_helper"
+
+describe SiteForm do
+  describe "validations" do
+    it "surfaces errors on the site model" do
+      site_form = build(:site_form, tna_timestamp: nil)
+
+      expect(site_form.valid?).to be false
+      expect(site_form.errors[:tna_timestamp]).to include("can't be blank")
+    end
+
+    it "surfaces errors on the default host model" do
+      site_form = build(:site_form, hostname: nil)
+
+      expect(site_form.valid?).to be false
+      expect(site_form.errors[:hostname]).to include("can't be blank")
+    end
+
+    describe "#aliases" do
+      context "when there is a validation error on an alias host" do
+        it "adds the error message and problematic hostname to the errors" do
+          site_form = build(:site_form, aliases: "aaib.gov.uk,,aaib.com")
+
+          expect(site_form.valid?).to be false
+          expect(site_form.errors[:aliases]).to include("\"\" can't be blank") # hostname is blank, hence ""
+        end
+      end
+
+      context "when the list contains duplicates" do
+        it "adds the error message and problematic hostname to the errors" do
+          site_form = build(:site_form, aliases: "aaib.gov.uk,aaib.gov.uk")
+
+          expect(site_form.valid?).to be false
+          expect(site_form.errors[:aliases]).to include("must be unique")
+        end
+      end
+    end
+  end
+
+  describe "#organisations" do
+    let!(:organisation) { create(:organisation, whitehall_slug: organisation_slug) }
+    let!(:other_organisation) { create(:organisation, whitehall_slug: "the-adjudicator-s-office", title: "The adjudicator's office") }
+    let(:organisation_slug) { "air-accidents-investigation-branch" }
+
+    it "returns all organisation except the current organisation" do
+      site_form = build(:site_form, organisation_slug:)
+
+      expect(site_form.organisations.length).to be 1
+      expect(site_form.organisations.last).to eq other_organisation
+    end
+  end
+
+  describe "#save" do
+    let!(:organisation) { create(:organisation, whitehall_slug: organisation_slug) }
+    let(:organisation_slug) { "air-accidents-investigation-branch" }
+
+    context "when invalid" do
+      it "returns false" do
+        site_form = build(:site_form, tna_timestamp: nil)
+
+        expect(site_form.save).to be false
+      end
+    end
+
+    it "returns an instance of the new Site" do
+      site_form = build(:site_form)
+
+      site = site_form.save
+
+      expect(site).to be_a Site
+      expect(site).to have_attributes(
+        abbr: "aaib",
+        organisation:,
+        homepage: "https://www.gov.uk/government/organisations/air-accidents-investigation-branch",
+        tna_timestamp: Time.strptime("20141104112824", "%Y%m%d%H%M%S"),
+      )
+    end
+
+    it "creates the default host and aka host" do
+      site_form = build(:site_form)
+
+      site = site_form.save
+
+      expect(site.hosts.count).to be 2
+      expect(site.hosts.first.hostname).to eq "www.aaib.gov.uk"
+      expect(site.hosts.last.hostname).to eq "aka.aaib.gov.uk"
+    end
+
+    context "with optional fields" do
+      it "returns an instance of the new Site" do
+        site_form = build(:site_form, :with_optional_fields)
+
+        site = site_form.save
+
+        expect(site).to be_a Site
+        expect(site).to have_attributes(
+          homepage_title: "Air accidents investigation branch",
+          homepage_furl: "www.gov.uk/aaib",
+          global_type: "redirect",
+          global_new_url: "https://www.gov.uk/government/organisations/air-accidents-investigation-branch/about",
+          global_redirect_append_path: true,
+          query_params: "file",
+          special_redirect_strategy: "via_aka",
+        )
+      end
+    end
+
+    context "with a comma-separated list of alias hosts" do
+      it "creates the alias hosts and aka hosts" do
+        site_form = build(:site_form, :with_aliases)
+
+        site = site_form.save
+
+        alias_site_1 = site.hosts.where(hostname: "aaib.gov.uk")
+        alias_site_2 = site.hosts.where(hostname: "aaib.com")
+
+        expect(alias_site_1).to exist
+        expect(alias_site_2).to exist
+        expect(site.hosts.where(hostname: "aka-aaib.gov.uk", canonical_host: alias_site_1)).to exist
+        expect(site.hosts.where(hostname: "aka-aaib.com", canonical_host: alias_site_2)).to exist
+      end
+    end
+
+    context "with extra organisations" do
+      let!(:extra_organisation) { create(:organisation, whitehall_slug: "the-adjudicator-s-office", title: "The adjudicator's office") }
+      let!(:extra_organisation_2) { create(:organisation, whitehall_slug: "government-digital-service", title: "Government digital service") }
+
+      it "creates extra organisations" do
+        site_form = build(:site_form, :with_extra_organisations)
+
+        site = site_form.save
+
+        expect(site.extra_organisations.count).to be 2
+        expect(site.extra_organisations.where(title: "The adjudicator's office")).to exist
+        expect(site.extra_organisations.where(title: "Government digital service")).to exist
+      end
+    end
+  end
+end
diff --git a/spec/models/host_spec.rb b/spec/models/host_spec.rb
index f9578fea1..9482f9520 100644
--- a/spec/models/host_spec.rb
+++ b/spec/models/host_spec.rb
@@ -75,6 +75,18 @@
           expect(host.errors_on(:hostname)).to include("is an invalid hostname")
         end
       end
+
+      context "is downcased" do
+        subject(:host) { build :host, hostname: "AB.com" }
+
+        describe "#valid?" do
+          subject { super().valid? }
+          it { is_expected.to be_falsey }
+        end
+        it "should have an error for invalid hostname" do
+          expect(host.errors_on(:hostname)).to include("must be lowercase")
+        end
+      end
     end
   end
 
diff --git a/spec/models/site_spec.rb b/spec/models/site_spec.rb
index f610da93f..2ac0fee66 100644
--- a/spec/models/site_spec.rb
+++ b/spec/models/site_spec.rb
@@ -41,6 +41,15 @@
       end
     end
 
+    context "global archive" do
+      subject(:site) { build(:site, global_type: "archive", global_new_url: "http://a.com/") }
+
+      before { expect(site).not_to be_valid }
+      it "should validate absence of global_new_url" do
+        expect(site.errors[:global_new_url]).to eq(["must be blank"])
+      end
+    end
+
     context "global redirect with path appended" do
       subject(:site) { build(:site, global_type: "redirect", global_redirect_append_path: true, global_new_url: "http://a.com/?") }
 
@@ -134,6 +143,40 @@
     end
   end
 
+  describe "nillifying blanks before validation" do
+    let(:site) { create :site, homepage_furl: "" }
+
+    subject { site.homepage_furl }
+
+    it { is_expected.to be_nil }
+
+    context "attributes not nilified" do
+      describe "#global_redirect_append_path" do
+        let(:site) { create :site, global_redirect_append_path: false }
+
+        subject { site.global_redirect_append_path }
+
+        it { is_expected.to be false }
+      end
+
+      describe "#global_redirect_append_path" do
+        let(:site) { create :site, query_params: "" }
+
+        subject { site.query_params }
+
+        it { is_expected.to eq "" }
+      end
+
+      describe "#precompute_all_hits_view" do
+        let(:site) { create :site, precompute_all_hits_view: false }
+
+        subject { site.precompute_all_hits_view }
+
+        it { is_expected.to be false }
+      end
+    end
+  end
+
   describe "#transition_status" do
     let!(:site) { create :site_without_host }
 
diff --git a/spec/requests/site_creation_spec.rb b/spec/requests/site_creation_spec.rb
new file mode 100644
index 000000000..2863d942b
--- /dev/null
+++ b/spec/requests/site_creation_spec.rb
@@ -0,0 +1,88 @@
+require "rails_helper"
+
+describe "Site creation" do
+  render_views
+
+  let!(:site_manager) { create(:site_manager) }
+  let(:organisation) { create(:organisation, whitehall_slug: "air-accidents-investigation-branch") }
+  let(:params) { attributes_for :site_form, :with_optional_fields, :with_aliases, organisation_slug: "air-accidents-investigation-branch" }
+
+  it "redirects to the new site path" do
+    post organisation_sites_path(organisation), params: { site_form: params }
+
+    expect(response).to redirect_to(site_path(Site.last))
+  end
+
+  it "creates the new site" do
+    post organisation_sites_path(organisation), params: { site_form: params }
+
+    attributes = {
+      abbr: "aaib",
+      global_new_url: "https://www.gov.uk/government/organisations/air-accidents-investigation-branch/about",
+      global_redirect_append_path: true,
+      global_type: "redirect",
+      homepage: "https://www.gov.uk/government/organisations/air-accidents-investigation-branch",
+      homepage_furl: "www.gov.uk/aaib",
+      homepage_title: "Air accidents investigation branch",
+      query_params: "file",
+      special_redirect_strategy: "via_aka",
+      tna_timestamp: Time.strptime("20141104112824", "%Y%m%d%H%M%S"),
+    }
+
+    expect(Site.last.attributes.with_indifferent_access).to include attributes
+    expect(Site.last.organisation).to eq organisation
+  end
+
+  it "creates related hosts and aka hosts" do
+    post organisation_sites_path(organisation), params: { site_form: params }
+
+    alias_site_1 = Host.where(hostname: "www.aaib.gov.uk")
+    alias_site_2 = Host.where(hostname: "aaib.gov.uk")
+    alias_site_3 = Host.where(hostname: "aaib.com")
+
+    expect(alias_site_1).to exist
+    expect(alias_site_2).to exist
+    expect(alias_site_3).to exist
+    expect(Host.where(hostname: "aka.aaib.gov.uk", canonical_host: alias_site_1)).to exist
+    expect(Host.where(hostname: "aka-aaib.gov.uk", canonical_host: alias_site_2)).to exist
+    expect(Host.where(hostname: "aka-aaib.com", canonical_host: alias_site_3)).to exist
+  end
+
+  context "with extra organisations" do
+    let!(:extra_organisation) { create(:organisation, whitehall_slug: "the-adjudicator-s-office", title: "The adjudicator's office") }
+    let!(:extra_organisation_2) { create(:organisation, whitehall_slug: "government-digital-service", title: "Government digital service") }
+    let(:params) { attributes_for(:site_form, :with_extra_organisations) }
+
+    it "creates extra organisations" do
+      post organisation_sites_path(organisation), params: { site_form: params }
+
+      expect(Site.last.extra_organisations.count).to be 2
+      expect(Site.last.extra_organisations.where(title: "The adjudicator's office")).to exist
+      expect(Site.last.extra_organisations.where(title: "Government digital service")).to exist
+    end
+  end
+
+  context "with blank optional fields" do
+    let(:params) { attributes_for(:site_form, :with_blank_optional_fields) }
+
+    it "nullifies blanks when creating the new site" do
+      post organisation_sites_path(organisation), params: { site_form: params }
+
+      attributes = {
+        abbr: "aaib",
+        homepage: "https://www.gov.uk/government/organisations/air-accidents-investigation-branch",
+        tna_timestamp: Time.strptime("20141104112824", "%Y%m%d%H%M%S"),
+        global_redirect_append_path: false,
+        global_new_url: nil,
+        special_redirect_strategy: nil,
+        global_type: nil,
+        homepage_title: nil,
+        homepage_furl: nil,
+        query_params: "",
+      }
+
+      expect(Site.last.attributes.with_indifferent_access).to include attributes
+      expect(Site.last.organisation).to eq organisation
+    end
+  end
+end
diff --git a/spec/support/shared_examples/authentication.rb b/spec/support/shared_examples/authentication.rb
index ad2c4bb67..ceffe4067 100644
--- a/spec/support/shared_examples/authentication.rb
+++ b/spec/support/shared_examples/authentication.rb
@@ -28,6 +28,21 @@
   end
 end
 
+shared_examples "disallows editing by non-Site managers" do
+  before do
+    login_as stub_user
+    make_request
+  end
+
+  it "redirects to the organisation page" do
+    expect(response).to redirect_to organisation_path(organisation)
+  end
+
+  it "sets a flash message" do
+    expect(flash[:alert]).to eql("Only Site Managers can access that.")
+  end
+end
+
 shared_examples "disallows editing of a global site" do
   before do
     make_request