diff --git a/app/models/validations/sales/sale_information_validations.rb b/app/models/validations/sales/sale_information_validations.rb
index 54d49b7e0..17fbc95b4 100644
--- a/app/models/validations/sales/sale_information_validations.rb
+++ b/app/models/validations/sales/sale_information_validations.rb
@@ -64,4 +64,23 @@ module Validations::Sales::SaleInformationValidations
       record.errors.add :grant, I18n.t("validations.sale_information.grant.out_of_range")
     end
   end
+
+  def validate_stairbought(record)
+    return unless record.stairbought && record.type
+    return unless record.saledate && record.form.start_year_after_2024?
+
+    max_stairbought = case record.type
+                      when 30, 16, 28, 31, 32
+                        90
+                      when 2, 18
+                        75
+                      when 24
+                        50
+                      end
+
+    if max_stairbought && record.stairbought > max_stairbought
+      record.errors.add :stairbought, I18n.t("validations.sale_information.stairbought.over_max", max_stairbought:, type: record.form.get_question("type", record).answer_label(record))
+      record.errors.add :type, I18n.t("validations.sale_information.stairbought.over_max", max_stairbought:, type: record.form.get_question("type", record).answer_label(record))
+    end
+  end
 end
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 7fc5f9d57..c7d8c9da2 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -617,6 +617,8 @@ en:
         higher_than_expected: "Basic monthly rent must be between £0.00 and £9,999.00"
       grant:
         out_of_range: "Loan, grants or subsidies must be between £9,000 and £16,000"
+      stairbought:
+        over_max: "The percentage bought in this staircasing transaction cannot be higher than %{max_stairbought}% for %{type} sales."
     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 c890afed1..c153d0742 100644
--- a/spec/models/validations/sales/sale_information_validations_spec.rb
+++ b/spec/models/validations/sales/sale_information_validations_spec.rb
@@ -539,4 +539,293 @@ RSpec.describe Validations::Sales::SaleInformationValidations do
       end
     end
   end
+
+  describe "#validate_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 ownership type is Shared Ownership (new model lease)" do
+      let(:record) { build(:sales_log, ownershipsch: 1, type: 30, saledate: now) }
+
+      it "does not add an error if stairbought is under 90%" do
+        record.stairbought = 89
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is 90%" do
+        record.stairbought = 90
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is not given" do
+        record.stairbought = nil
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "adds an error if stairbought is over 90%" do
+        record.stairbought = 92
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors[:stairbought]).to include("The percentage bought in this staircasing transaction cannot be higher than 90% for Shared Ownership (new model lease) sales.")
+        expect(record.errors[:type]).to include("The percentage bought in this staircasing transaction cannot be higher than 90% for Shared Ownership (new model lease) sales.")
+      end
+    end
+
+    context "when ownership type is Home ownership for people with Long-Term Disabilities (HOLD)" do
+      let(:record) { build(:sales_log, ownershipsch: 1, type: 16, saledate: now) }
+
+      it "does not add an error if stairbought is under 90%" do
+        record.stairbought = 89
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is 90%" do
+        record.stairbought = 90
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is not given" do
+        record.stairbought = nil
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "adds an error if stairbought is over 90%" do
+        record.stairbought = 92
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors[:stairbought]).to include("The percentage bought in this staircasing transaction cannot be higher than 90% for Home Ownership for people with Long-Term Disabilities (HOLD) sales.")
+        expect(record.errors[:type]).to include("The percentage bought in this staircasing transaction cannot be higher than 90% for Home Ownership for people with Long-Term Disabilities (HOLD) sales.")
+      end
+    end
+
+    context "when ownership type is Rent to Buy – shared ownership" do
+      let(:record) { build(:sales_log, ownershipsch: 1, type: 28, saledate: now) }
+
+      it "does not add an error if stairbought is under 90%" do
+        record.stairbought = 89
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is 90%" do
+        record.stairbought = 90
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is not given" do
+        record.stairbought = nil
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "adds an error if stairbought is over 90%" do
+        record.stairbought = 92
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors[:stairbought]).to include("The percentage bought in this staircasing transaction cannot be higher than 90% for Rent to Buy — Shared Ownership sales.")
+        expect(record.errors[:type]).to include("The percentage bought in this staircasing transaction cannot be higher than 90% for Rent to Buy — Shared Ownership sales.")
+      end
+    end
+
+    context "when ownership type is Right to Shared Ownership" do
+      let(:record) { build(:sales_log, ownershipsch: 1, type: 31, saledate: now) }
+
+      it "does not add an error if stairbought is under 90%" do
+        record.stairbought = 89
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is 90%" do
+        record.stairbought = 90
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is not given" do
+        record.stairbought = nil
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "adds an error if stairbought is over 90%" do
+        record.stairbought = 92
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors[:stairbought]).to include("The percentage bought in this staircasing transaction cannot be higher than 90% for Right to Shared Ownership (RtSO) sales.")
+        expect(record.errors[:type]).to include("The percentage bought in this staircasing transaction cannot be higher than 90% for Right to Shared Ownership (RtSO) sales.")
+      end
+    end
+
+    context "when ownership type is London Living Rent – shared ownership" do
+      let(:record) { build(:sales_log, ownershipsch: 1, type: 32, saledate: now) }
+
+      it "does not add an error if stairbought is under 90%" do
+        record.stairbought = 89
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is 90%" do
+        record.stairbought = 90
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is not given" do
+        record.stairbought = nil
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "adds an error if stairbought is over 90%" do
+        record.stairbought = 92
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors[:stairbought]).to include("The percentage bought in this staircasing transaction cannot be higher than 90% for London Living Rent — Shared Ownership sales.")
+        expect(record.errors[:type]).to include("The percentage bought in this staircasing transaction cannot be higher than 90% for London Living Rent — Shared Ownership sales.")
+      end
+    end
+
+    context "when ownership type is Shared Ownership (old model lease)" do
+      let(:record) { build(:sales_log, ownershipsch: 1, type: 2, saledate: now) }
+
+      it "does not add an error if stairbought is under 75%" do
+        record.stairbought = 60
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is 75%" do
+        record.stairbought = 75
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is not given" do
+        record.stairbought = nil
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "adds an error if stairbought is over 75%" do
+        record.stairbought = 76
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors[:stairbought]).to include("The percentage bought in this staircasing transaction cannot be higher than 75% for Shared Ownership (old model lease) sales.")
+        expect(record.errors[:type]).to include("The percentage bought in this staircasing transaction cannot be higher than 75% for Shared Ownership (old model lease) sales.")
+      end
+    end
+
+    context "when ownership type is Social Homebuy – shared ownership" do
+      let(:record) { build(:sales_log, ownershipsch: 1, type: 18, saledate: now) }
+
+      it "does not add an error if stairbought is under 75%" do
+        record.stairbought = 60
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is 75%" do
+        record.stairbought = 75
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is not given" do
+        record.stairbought = nil
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "adds an error if stairbought is over 75%" do
+        record.stairbought = 76
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors[:stairbought]).to include("The percentage bought in this staircasing transaction cannot be higher than 75% for Social HomeBuy — shared ownership purchase sales.")
+        expect(record.errors[:type]).to include("The percentage bought in this staircasing transaction cannot be higher than 75% for Social HomeBuy — shared ownership purchase sales.")
+      end
+    end
+
+    context "when ownership type is Older Persons shared ownership (OPSO)" do
+      let(:record) { build(:sales_log, ownershipsch: 1, type: 24, saledate: now) }
+
+      it "does not add an error if stairbought is under 50%" do
+        record.stairbought = 33
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is 50%" do
+        record.stairbought = 50
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "does not add an error if stairbought is not given" do
+        record.stairbought = nil
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+
+      it "adds an error if stairbought is over 50%" do
+        record.stairbought = 55
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors[:stairbought]).to include("The percentage bought in this staircasing transaction cannot be higher than 50% for Older Persons Shared Ownership sales.")
+        expect(record.errors[:type]).to include("The percentage bought in this staircasing transaction cannot be higher than 50% for Older Persons Shared Ownership sales.")
+      end
+    end
+
+    context "when the collection year is before 2024" do
+      let(:record) { build(:sales_log, ownershipsch: 1, type: 24, saledate: now, stairbought: 90) }
+      let(:now) { Time.zone.local(2023, 4, 4) }
+
+      it "does not add an error" do
+        sale_information_validator.validate_stairbought(record)
+
+        expect(record.errors).to be_empty
+      end
+    end
+  end
 end