diff --git a/app/models/sales_log.rb b/app/models/sales_log.rb
index 4de9699ba..13d16a786 100644
--- a/app/models/sales_log.rb
+++ b/app/models/sales_log.rb
@@ -496,7 +496,7 @@ class SalesLog < Log
   def is_not_staircasing?
     staircase == 2 || staircase == 3
   end
-  
+
   def stairowned_100?
     stairowned == 100
   end
diff --git a/app/models/validations/sales/sale_information_validations.rb b/app/models/validations/sales/sale_information_validations.rb
index 0aaa77a93..c8a4e46dc 100644
--- a/app/models/validations/sales/sale_information_validations.rb
+++ b/app/models/validations/sales/sale_information_validations.rb
@@ -110,4 +110,14 @@ module Validations::Sales::SaleInformationValidations
       end
     end
   end
+
+  def validate_mortgage_used_and_stairbought(record)
+    return unless record.stairowned && record.mortgageused
+    return unless record.saledate && record.form.start_year_after_2024?
+
+    if !record.stairowned_100? && record.mortgageused == 3
+      record.errors.add :stairowned, I18n.t("validations.sale_information.stairowned.mortgageused_dont_know")
+      record.errors.add :mortgageused, I18n.t("validations.sale_information.stairowned.mortgageused_dont_know")
+    end
+  end
 end
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 204a20b4e..37239d234 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -623,6 +623,8 @@ en:
         over_discounted_london_max: "The percentage discount multiplied by the purchase price is %{discount_value}. This figure should not be more than £136,400 for properties in London."
         over_discounted_max: "The percentage discount multiplied by the purchase price is %{discount_value}. This figure should not be more than £102,400 for properties outside of London."
       non_staircasing_mortgage: "The mortgage and deposit added together is %{mortgage_and_deposit_total} and the purchase price times by the equity is %{expected_shared_ownership_deposit_value}. These figures should be the same."
+      stairowned:
+        mortgageused_dont_know: "The percentage owned has to be 100% if the mortgage used is 'Don’t know'"
     merge_request:
       organisation_part_of_another_merge: "This organisation is part of another merge - select a different one"
       organisation_not_selected: "Select an organisation from the search list"
diff --git a/spec/models/validations/sales/sale_information_validations_spec.rb b/spec/models/validations/sales/sale_information_validations_spec.rb
index 73243e9fa..3d323112e 100644
--- a/spec/models/validations/sales/sale_information_validations_spec.rb
+++ b/spec/models/validations/sales/sale_information_validations_spec.rb
@@ -756,4 +756,48 @@ RSpec.describe Validations::Sales::SaleInformationValidations do
       end
     end
   end
+
+  describe "#validate_mortgage_used_and_stairbought" do
+    let(:now) { Time.zone.local(2024, 4, 4) }
+
+    before do
+      Timecop.freeze(now)
+      Singleton.__init__(FormHandler)
+    end
+
+    after do
+      Timecop.return
+      Singleton.__init__(FormHandler)
+    end
+
+    context "when mortgageused don't know" do
+      let(:record) { build(:sales_log, ownershipsch: 1, type: 9, saledate: now, mortgageused: 3) }
+
+      it "does not add an error if stairowned 100" do
+        record.stairowned = 100
+        sale_information_validator.validate_mortgage_used_and_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "adds an error if stairowned is not 100" do
+        record.stairowned = 90
+        sale_information_validator.validate_mortgage_used_and_stairbought(record)
+
+        expect(record.errors[:stairowned]).to include("The percentage owned has to be 100% if the mortgage used is 'Don’t know'")
+        expect(record.errors[:mortgageused]).to include("The percentage owned has to be 100% if the mortgage used is 'Don’t know'")
+      end
+    end
+
+    context "when the collection year is before 2024" do
+      let(:record) { build(:sales_log, ownershipsch: 1, type: 9, saledate: now, mortgageused: 3, stairowned: 90) }
+      let(:now) { Time.zone.local(2023, 4, 4) }
+
+      it "does not add an error" do
+        sale_information_validator.validate_mortgage_used_and_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+    end
+  end
 end