diff --git a/app/models/sales_log.rb b/app/models/sales_log.rb
index 654808d3a..616ce64ae 100644
--- a/app/models/sales_log.rb
+++ b/app/models/sales_log.rb
@@ -268,6 +268,12 @@ class SalesLog < Log
     value * equity / 100
   end
 
+  def expected_shared_ownership_deposit_value_range
+    return unless value && equity
+
+    (value * ((equity - 0.1) / 100)..value * ((equity + 0.1) / 100))
+  end
+
   def stairbought_part_of_value
     return unless value && stairbought
 
@@ -468,6 +474,15 @@ class SalesLog < Log
     value - discount_amount
   end
 
+  def value_with_discount_range
+    return if value.blank?
+    return (value..value) if discount.nil?
+
+    discount_amount_lower_bound = value * (discount - 0.1) / 100
+    discount_amount_upper_bound = value * (discount + 0.1) / 100
+    (value - discount_amount_upper_bound..value - discount_amount_lower_bound)
+  end
+
   def mortgage_deposit_and_grant_total
     return if deposit.blank?
 
diff --git a/app/models/validations/sales/sale_information_validations.rb b/app/models/validations/sales/sale_information_validations.rb
index 063c16599..399deb724 100644
--- a/app/models/validations/sales/sale_information_validations.rb
+++ b/app/models/validations/sales/sale_information_validations.rb
@@ -80,10 +80,7 @@ module Validations::Sales::SaleInformationValidations
     return unless record.mortgage || record.mortgageused == 2 || record.mortgageused == 3
     return unless record.discount || record.grant || record.type == 29
 
-    # When a percentage discount is used, a percentage tolerance is needed to account for rounding errors
-    tolerance = record.discount ? record.value * 0.05 / 100 : 1
-
-    if over_tolerance?(record.mortgage_deposit_and_grant_total, record.value_with_discount, tolerance, strict: !record.discount.nil?) && record.discounted_ownership_sale?
+    if mortgage_deposit_and_grant_total_is_over_discount_tolerance?(record) && record.discounted_ownership_sale?
       deposit_and_grant_sentence = record.grant.present? ? ", cash deposit (#{record.field_formatted_as_currency('deposit')}), and grant (#{record.field_formatted_as_currency('grant')})" : " and cash deposit (#{record.field_formatted_as_currency('deposit')})"
       discount_sentence = record.discount.present? ? " (#{record.field_formatted_as_currency('value')}) subtracted by the sum of the full purchase price (#{record.field_formatted_as_currency('value')}) multiplied by the percentage discount (#{record.discount}%)" : ""
       %i[mortgageused mortgage value deposit discount grant].each do |field|
@@ -207,7 +204,7 @@ module Validations::Sales::SaleInformationValidations
     if record.mortgage_used?
       return unless record.mortgage
 
-      if over_tolerance?(record.mortgage_deposit_and_discount_total, record.expected_shared_ownership_deposit_value, 1)
+      if total_is_over_expected_shared_ownership_deposit_tolerance?(record.mortgage_deposit_and_discount_total, record)
         %i[mortgage value deposit cashdis equity].each do |field|
           record.errors.add field, I18n.t("validations.sales.sale_information.#{field}.non_staircasing_mortgage.mortgage_used_socialhomebuy",
                                           mortgage: record.field_formatted_as_currency("mortgage"),
@@ -228,7 +225,7 @@ module Validations::Sales::SaleInformationValidations
                                                                  expected_shared_ownership_deposit_value: record.field_formatted_as_currency("expected_shared_ownership_deposit_value")).html_safe
       end
     elsif record.mortgage_not_used?
-      if over_tolerance?(record.deposit_and_discount_total, record.expected_shared_ownership_deposit_value, 1)
+      if total_is_over_expected_shared_ownership_deposit_tolerance?(record.deposit_and_discount_total, record)
         %i[mortgageused value deposit cashdis equity].each do |field|
           record.errors.add field, I18n.t("validations.sales.sale_information.#{field}.non_staircasing_mortgage.mortgage_not_used_socialhomebuy",
                                           deposit_and_discount_total: record.field_formatted_as_currency("deposit_and_discount_total"),
@@ -253,7 +250,7 @@ module Validations::Sales::SaleInformationValidations
     if record.mortgage_used?
       return unless record.mortgage
 
-      if over_tolerance?(record.mortgage_and_deposit_total, record.expected_shared_ownership_deposit_value, 1)
+      if total_is_over_expected_shared_ownership_deposit_tolerance?(record.mortgage_and_deposit_total, record)
         %i[mortgage value deposit equity].each do |field|
           record.errors.add field, I18n.t("validations.sales.sale_information.#{field}.non_staircasing_mortgage.mortgage_used",
                                           mortgage: record.field_formatted_as_currency("mortgage"),
@@ -272,7 +269,7 @@ module Validations::Sales::SaleInformationValidations
                                                                  expected_shared_ownership_deposit_value: record.field_formatted_as_currency("expected_shared_ownership_deposit_value")).html_safe
       end
     elsif record.mortgage_not_used?
-      if over_tolerance?(record.deposit, record.expected_shared_ownership_deposit_value, 1)
+      if total_is_over_expected_shared_ownership_deposit_tolerance?(record.deposit, record)
         %i[mortgageused value deposit equity].each do |field|
           record.errors.add field, I18n.t("validations.sales.sale_information.#{field}.non_staircasing_mortgage.mortgage_not_used",
                                           deposit: record.field_formatted_as_currency("deposit"),
@@ -396,6 +393,25 @@ module Validations::Sales::SaleInformationValidations
     end
   end
 
+  def total_is_over_expected_shared_ownership_deposit_tolerance?(total, record)
+    if record.form.start_year_2026_or_later?
+      !record.expected_shared_ownership_deposit_value_range.cover?(total)
+    else
+      over_tolerance?(total, record.expected_shared_ownership_deposit_value, 1)
+    end
+  end
+
+  def mortgage_deposit_and_grant_total_is_over_discount_tolerance?(record)
+    if record.form.start_year_2026_or_later?
+      !record.value_with_discount_range.cover?(record.mortgage_deposit_and_grant_total)
+    else
+      # we found that this tolerance still wasn't working for us, so for post 2026 we just calculate a range for 0.1% higher and lower.
+      # this means we can be certain an answer 0.1% higher or lower will always be accepted without needed to backsolve for the tolerance
+      tolerance = record.discount ? record.value * 0.05 / 100 : 1
+      over_tolerance?(record.mortgage_deposit_and_grant_total, record.value_with_discount, tolerance, strict: record.discount.present?)
+    end
+  end
+
   def over_tolerance?(expected, actual, tolerance, strict: false)
     if strict
       (expected - actual).abs > tolerance