diff --git a/app/models/validations/sales/financial_validations.rb b/app/models/validations/sales/financial_validations.rb
index f66618e16..9e30d49f3 100644
--- a/app/models/validations/sales/financial_validations.rb
+++ b/app/models/validations/sales/financial_validations.rb
@@ -119,6 +119,18 @@ module Validations::Sales::FinancialValidations
     end
   end
 
+  def validate_equity_less_than_staircase_difference(record)
+    return unless record.equity && record.stairbought && record.stairowned
+    return unless record.saledate && record.form.start_year_after_2024?
+
+    if record.equity > record.stairowned - record.stairbought
+      formatted_equity = sprintf("%g", record.equity)
+      record.errors.add :equity, I18n.t("validations.financial.equity.over_stairowned_minus_stairbought", equity: formatted_equity, staircase_difference: record.stairowned - record.stairbought)
+      record.errors.add :stairowned, I18n.t("validations.financial.equity.over_stairowned_minus_stairbought", equity: formatted_equity, staircase_difference: record.stairowned - record.stairbought)
+      record.errors.add :stairbought, I18n.t("validations.financial.equity.over_stairowned_minus_stairbought", equity: formatted_equity, staircase_difference: record.stairowned - record.stairbought)
+    end
+  end
+
 private
 
   def is_relationship_child?(relationship)
diff --git a/config/locales/en.yml b/config/locales/en.yml
index e04d9465b..cdb3ef8d7 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -433,6 +433,7 @@ en:
       equity:
         under_min: "The minimum initial equity stake for this type of shared ownership sale is %{min_equity}%"
         over_max: "The maximum initial equity stake is %{max_equity}%"
+        over_stairowned_minus_stairbought: "The initial equity stake is %{equity}% and the percentage owned in total minus the percentage bought is %{staircase_difference}%. In a staircasing transaction, the equity stake purchased cannot be larger than the percentage the buyer owns minus the percentage bought."
       mortgage: "Mortgage value cannot be £0 if a mortgage was used for the purchase of this property"
       shared_ownership_deposit: "The %{mortgage_deposit_and_discount_error_fields} added together is %{mortgage_deposit_and_discount_total}. The value times the equity percentage is %{value_times_equity}. These figures should be the same"
 
diff --git a/spec/models/validations/sales/financial_validations_spec.rb b/spec/models/validations/sales/financial_validations_spec.rb
index 00dc9f850..8a57df6b6 100644
--- a/spec/models/validations/sales/financial_validations_spec.rb
+++ b/spec/models/validations/sales/financial_validations_spec.rb
@@ -561,4 +561,83 @@ RSpec.describe Validations::Sales::FinancialValidations do
       end
     end
   end
+
+  describe "#validate_equity_less_than_staircase_difference" do
+    let(:record) { FactoryBot.create(:sales_log, saledate: now) }
+
+    around do |example|
+      Timecop.freeze(now) do
+        Singleton.__init__(FormHandler)
+        example.run
+      end
+      Timecop.return
+      Singleton.__init__(FormHandler)
+    end
+
+    context "with a log in the 23/24 collection year" do
+      let(:now) { Time.zone.local(2023, 4, 1) }
+
+      it "does not add an error" do
+        record.stairbought = 2
+        record.stairowned = 3
+        record.equity = 2
+        financial_validator.validate_equity_less_than_staircase_difference(record)
+        expect(record.errors).to be_empty
+      end
+    end
+
+    context "with a log in 24/25 collection year" do
+      let(:now) { Time.zone.local(2024, 4, 1) }
+
+      it "adds errors if equity is more than stairowned - stairbought" do
+        record.stairbought = 2
+        record.stairowned = 3
+        record.equity = 2
+        financial_validator.validate_equity_less_than_staircase_difference(record)
+        expect(record.errors["equity"]).to include("The initial equity stake is 2% and the percentage owned in total minus the percentage bought is 1%. In a staircasing transaction, the equity stake purchased cannot be larger than the percentage the buyer owns minus the percentage bought.")
+        expect(record.errors["stairowned"]).to include("The initial equity stake is 2% and the percentage owned in total minus the percentage bought is 1%. In a staircasing transaction, the equity stake purchased cannot be larger than the percentage the buyer owns minus the percentage bought.")
+        expect(record.errors["stairbought"]).to include("The initial equity stake is 2% and the percentage owned in total minus the percentage bought is 1%. In a staircasing transaction, the equity stake purchased cannot be larger than the percentage the buyer owns minus the percentage bought.")
+      end
+
+      it "does not add errors if equity is less than stairowned - stairbought" do
+        record.stairbought = 2
+        record.stairowned = 10
+        record.equity = 2
+        financial_validator.validate_equity_less_than_staircase_difference(record)
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add errors if equity is equal stairowned - stairbought" do
+        record.stairbought = 2
+        record.stairowned = 10
+        record.equity = 8
+        financial_validator.validate_equity_less_than_staircase_difference(record)
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add errors if stairbought is not given" do
+        record.stairbought = nil
+        record.stairowned = 10
+        record.equity = 2
+        financial_validator.validate_equity_less_than_staircase_difference(record)
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add errors if stairowned is not given" do
+        record.stairbought = 2
+        record.stairowned = nil
+        record.equity = 2
+        financial_validator.validate_equity_less_than_staircase_difference(record)
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add errors if equity is not given" do
+        record.stairbought = 2
+        record.stairowned = 10
+        record.equity = 0
+        financial_validator.validate_equity_less_than_staircase_difference(record)
+        expect(record.errors).to be_empty
+      end
+    end
+  end
 end