diff --git a/app/models/validations/sales/sale_information_validations.rb b/app/models/validations/sales/sale_information_validations.rb
index 32bf2a716..cec4a16ff 100644
--- a/app/models/validations/sales/sale_information_validations.rb
+++ b/app/models/validations/sales/sale_information_validations.rb
@@ -370,6 +370,21 @@ module Validations::Sales::SaleInformationValidations
     end
   end
 
+  def validate_staircasing_purchase_price(record)
+    return unless record.form.start_year_2026_or_later?
+    return unless record.is_staircase? && record.value && record.stairbought&.positive?
+
+    full_purchase_price = record.value * 100 / record.stairbought
+    if full_purchase_price < 15_000
+      %i[value stairbought].each do |field|
+        record.errors.add field, I18n.t("validations.sales.sale_information.#{field}.full_purchase_price_below_min",
+                                        value: record.field_formatted_as_currency("value"),
+                                        stairbought: record.stairbought,
+                                        full_purchase_price: format_as_currency(full_purchase_price))
+      end
+    end
+  end
+
   def validate_mortgage_used_dont_know(record)
     return unless record.mortgage_use_unknown?
     return if record.form.start_year_2026_or_later?
diff --git a/config/locales/validations/sales/sale_information.en.yml b/config/locales/validations/sales/sale_information.en.yml
index 3c39608cc..9d932ade4 100644
--- a/config/locales/validations/sales/sale_information.en.yml
+++ b/config/locales/validations/sales/sale_information.en.yml
@@ -54,6 +54,7 @@ en:
             mortgage_used: "The mortgage (%{mortgage}) and cash deposit (%{deposit}) added together is %{mortgage_and_deposit_total}.</br></br>The full purchase price (%{value}) multiplied by the percentage bought is %{stairbought_part_of_value}.</br></br>These two amounts should be the same."
             mortgage_used_socialhomebuy: "The mortgage amount (%{mortgage}), cash deposit (%{deposit}), and cash discount (%{cashdis}) added together is %{mortgage_deposit_and_discount_total}.</br></br>The full purchase price (%{value}) multiplied by the percentage bought (%{stairbought}) is %{stairbought_part_of_value}.</br></br>These two amounts should be the same."
         value:
+          full_purchase_price_below_min: "Based on the purchase price of %{value} and the percentage bought of %{stairbought}%, the implied full property value is %{full_purchase_price}. The full property value must be at least £15,000."
           discounted_ownership_value: "The mortgage%{mortgage}%{deposit_and_grant_sentence} added together is %{mortgage_deposit_and_grant_total}.</br></br>The full purchase price%{discount_sentence} is %{value_with_discount}.</br></br>These two amounts should be the same."
           outright_sale_value: "The mortgage%{mortgage} and cash deposit (%{deposit}) when added together is %{mortgage_and_deposit_total}.</br></br>The full purchase price is %{value}.</br></br>These two amounts should be the same."
           value_over_discounted_london_max: "The percentage discount multiplied by the purchase price is %{discount_value}. This figure should not be more than £137,400 for properties in London."
@@ -76,7 +77,7 @@ en:
             mortgage_not_used: "The cash deposit is %{deposit}.</br></br>The full purchase price (%{value}) multiplied by the percentage bought is %{expected_shared_ownership_deposit_value}.</br></br>These two amounts should be the same."
             mortgage_used_socialhomebuy: "The mortgage amount (%{mortgage}), cash deposit (%{deposit}), and cash discount (%{cashdis}) added together is %{mortgage_deposit_and_discount_total}.</br></br>The full purchase price (%{value}) multiplied by the percentage equity stake purchased (%{equity}) is %{expected_shared_ownership_deposit_value}.</br></br>These two amounts should be the same."
             mortgage_not_used_socialhomebuy: "The cash deposit (%{deposit}) and cash discount (%{cashdis}) added together is %{deposit_and_discount_total}.</br></br>The full purchase price (%{value}) multiplied by the percentage bought (%{equity}) is %{expected_shared_ownership_deposit_value}.</br></br>These two amounts should be the same."
-          staircasing_mortgage: # this key
+          staircasing_mortgage:
             mortgage_used: "The mortgage (%{mortgage}) and cash deposit (%{deposit}) added together is %{mortgage_and_deposit_total}.</br></br>The full purchase price (%{value}) multiplied by the percentage bought is %{stairbought_part_of_value}.</br></br>These two amounts should be the same."
             mortgage_not_used: "The cash deposit is %{deposit}.</br></br>The full purchase price (%{value}) multiplied by the percentage bought is %{stairbought_part_of_value}.</br></br>These two amounts should be the same."
             mortgage_used_socialhomebuy: "The mortgage amount (%{mortgage}), cash deposit (%{deposit}), and cash discount (%{cashdis}) added together is %{mortgage_deposit_and_discount_total}.</br></br>The full purchase price (%{value}) multiplied by the percentage bought (%{stairbought}) is %{stairbought_part_of_value}.</br></br>These two amounts should be the same."
@@ -117,6 +118,7 @@ en:
             mortgage_used_socialhomebuy: "The mortgage amount (%{mortgage}), cash deposit (%{deposit}), and cash discount (%{cashdis}) added together is %{mortgage_deposit_and_discount_total}.</br></br>The full purchase price (%{value}) multiplied by the percentage bought (%{stairbought}) is %{stairbought_part_of_value}.</br></br>These two amounts should be the same."
             mortgage_not_used_socialhomebuy: "The cash deposit (%{deposit}) and cash discount (%{cashdis}) added together is %{deposit_and_discount_total}.</br></br>The full purchase price (%{value}) multiplied by the percentage bought (%{stairbought}) is %{stairbought_part_of_value}.</br></br>These two amounts should be the same."
         stairbought:
+          full_purchase_price_below_min: "Based on the purchase price of %{value} and the percentage bought of %{stairbought}%, the implied full property value is %{full_purchase_price}. The full property value must be at least £15,000."
           stairbought_over_max: "The percentage bought in this staircasing transaction cannot be higher than %{max_stairbought}% for %{type} sales."
           staircasing_mortgage:
             mortgage_used: "The mortgage (%{mortgage}) and cash deposit (%{deposit}) added together is %{mortgage_and_deposit_total}.</br></br>The full purchase price (%{value}) multiplied by the percentage bought is %{stairbought_part_of_value}.</br></br>These two amounts should be the same."
diff --git a/spec/models/validations/sales/sale_information_validations_spec.rb b/spec/models/validations/sales/sale_information_validations_spec.rb
index 8d4eb4cea..446a6e033 100644
--- a/spec/models/validations/sales/sale_information_validations_spec.rb
+++ b/spec/models/validations/sales/sale_information_validations_spec.rb
@@ -994,6 +994,62 @@ RSpec.describe Validations::Sales::SaleInformationValidations do
     end
   end
 
+  describe "#validate_staircasing_purchase_price" do
+    let(:saledate) { Time.zone.local(2026, 4, 1) }
+    let(:record) { build(:sales_log, ownershipsch: 1, staircase: 1, saledate:) }
+
+    it "does not add an error when not staircasing" do
+      record.staircase = 2
+      record.value = 5_000
+      record.stairbought = 10
+      sale_information_validator.validate_staircasing_purchase_price(record)
+
+      expect(record.errors).to be_empty
+    end
+
+    it "does not add an error when value is nil" do
+      record.value = nil
+      record.stairbought = 10
+      sale_information_validator.validate_staircasing_purchase_price(record)
+
+      expect(record.errors).to be_empty
+    end
+
+    it "does not add an error when stairbought is nil" do
+      record.value = 5_000
+      record.stairbought = nil
+      sale_information_validator.validate_staircasing_purchase_price(record)
+
+      expect(record.errors).to be_empty
+    end
+
+    it "does not add an error when full purchase price is at least 15,000" do
+      record.value = 1_500
+      record.stairbought = 10
+      sale_information_validator.validate_staircasing_purchase_price(record)
+
+      expect(record.errors).to be_empty
+    end
+
+    it "adds an error when full purchase price is below 15,000" do
+      record.value = 1_000
+      record.stairbought = 10
+      sale_information_validator.validate_staircasing_purchase_price(record)
+
+      expect(record.errors[:value]).to be_present
+      expect(record.errors[:stairbought]).to be_present
+    end
+
+    it "does not add an error for years before 2026" do
+      record.saledate = Time.zone.local(2025, 4, 1)
+      record.value = 1_000
+      record.stairbought = 10
+      sale_information_validator.validate_staircasing_purchase_price(record)
+
+      expect(record.errors).to be_empty
+    end
+  end
+
   describe "#validate_discount_and_value" do
     let(:record) { FactoryBot.build(:sales_log, value: 200_000, discount: 50, ownershipsch: 2, type: 9, saledate:) }
     let(:saledate) { current_collection_start_date }