diff --git a/app/models/derived_variables/lettings_log_variables.rb b/app/models/derived_variables/lettings_log_variables.rb
index 1c7f161c1..68fa67438 100644
--- a/app/models/derived_variables/lettings_log_variables.rb
+++ b/app/models/derived_variables/lettings_log_variables.rb
@@ -75,6 +75,10 @@ module DerivedVariables::LettingsLogVariables
       self.beds = nil
     end
 
+    if form.start_year_2026_or_later?
+      infer_at_most_one_relationship!
+    end
+
     clear_child_constraints_for_age_changes!
     child_under_16_constraints!
 
@@ -295,6 +299,23 @@ private
     end
   end
 
+  def infer_at_most_one_relationship!
+    new_partner_numbers = partner_numbers.select { |i| public_send("relat#{i}_changed?") }
+    if new_partner_numbers.any?
+      infer_only_partner!(new_partner_numbers.first)
+    elsif partner_numbers.any?
+      infer_only_partner!(partner_numbers.first)
+    end
+  end
+
+  def infer_only_partner!(partner_number)
+    other_partner_numbers = partner_numbers.reject { |x| x == partner_number }
+    other_partner_numbers.each { |i| self["relat#{i}"] = "X" }
+
+    unanswered_partner_questions = (2..8).select { |i| public_send("relat#{i}").nil? }
+    unanswered_partner_questions.each { |i| self["relat#{i}"] = "X" }
+  end
+
   def household_type
     return unless totelder && totadult && totchild
 
diff --git a/app/models/form/lettings/pages/person_lead_partner.rb b/app/models/form/lettings/pages/person_lead_partner.rb
index 36f2e9b11..17d63149c 100644
--- a/app/models/form/lettings/pages/person_lead_partner.rb
+++ b/app/models/form/lettings/pages/person_lead_partner.rb
@@ -18,8 +18,11 @@ class Form::Lettings::Pages::PersonLeadPartner < ::Form::Page
             "operator" => ">=",
             "operand" => 16,
           },
+          **(2...@person_index).map { |i| ["relat#{i}", { "operator" => "!=", "operand" => "P" }] }.to_h,
         },
-        { "details_known_#{@person_index}" => 0, "age#{@person_index}" => nil },
+        { "details_known_#{@person_index}" => 0,
+          "age#{@person_index}" => nil,
+          **(2...@person_index).map { |i| ["relat#{i}", { "operator" => "!=", "operand" => "P" }] }.to_h },
       ]
     else
       [{ "details_known_#{@person_index}" => 0 }]
diff --git a/app/models/form/lettings/questions/person_partner.rb b/app/models/form/lettings/questions/person_partner.rb
index 98a22a448..7a09ca8cd 100644
--- a/app/models/form/lettings/questions/person_partner.rb
+++ b/app/models/form/lettings/questions/person_partner.rb
@@ -7,6 +7,12 @@ class Form::Lettings::Questions::PersonPartner < ::Form::Question
     @answer_options = answer_options
     @person_index = person_index
     @question_number = question_number
+    @hidden_in_check_answers = {
+      "depends_on" => (2...person_index).map do |i|
+        { "relat#{i}" => "P", "relat#{person_index}" => "X" }
+      end,
+    }
+    @disable_clearing_if_not_routed_or_dynamic_answer_options = form.start_year_2026_or_later?
   end
 
   def answer_options
diff --git a/app/models/form/lettings/subsections/household_characteristics.rb b/app/models/form/lettings/subsections/household_characteristics.rb
index 074709f8d..b87897e3b 100644
--- a/app/models/form/lettings/subsections/household_characteristics.rb
+++ b/app/models/form/lettings/subsections/household_characteristics.rb
@@ -35,8 +35,8 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
       Form::Lettings::Pages::PersonKnown.new(nil, nil, self, person_index: 2),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 2) if form.start_year_2026_or_later?),
       relationship_question(person_index: 2),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_2_partner_under_16_value_check", nil, self, person_index: 2) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
-      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_2_multiple_partners_value_check", nil, self, person_index: 2) if form.start_year_2024_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_2_partner_under_16_value_check", nil, self, person_index: 2) unless form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_2_multiple_partners_value_check", nil, self, person_index: 2) unless form.start_year_2026_or_later?),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 2) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonAgeValueCheck.new(nil, nil, self,
                                                                                person_index: 2),
@@ -44,7 +44,7 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
                                                                                              person_index: 2),
       Form::Lettings::Pages::PersonUnderRetirementValueCheck.new("age_2_under_retirement_value_check", nil, self, person_index: 2),
       Form::Lettings::Pages::PersonOverRetirementValueCheck.new("age_2_over_retirement_value_check", nil, self, person_index: 2),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_2_partner_under_16_value_check", nil, self, person_index: 2) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_2_partner_under_16_value_check", nil, self, person_index: 2) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::PersonGenderIdentity.new(nil, nil, self, person_index: 2),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonValueCheck.new(nil, nil, self, person_index: 2),
       Form::Lettings::Pages::FemalesInSoftAgeRangeInPregnantHouseholdPersonValueCheck.new(nil, nil, self,
@@ -57,8 +57,8 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
       Form::Lettings::Pages::PersonKnown.new(nil, nil, self, person_index: 3),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 3) if form.start_year_2026_or_later?),
       relationship_question(person_index: 3),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_3_partner_under_16_value_check", nil, self, person_index: 3) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
-      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_3_multiple_partners_value_check", nil, self, person_index: 3) if form.start_year_2024_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_3_partner_under_16_value_check", nil, self, person_index: 3) unless form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_3_multiple_partners_value_check", nil, self, person_index: 3) unless form.start_year_2026_or_later?),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 3) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonAgeValueCheck.new(nil, nil, self,
                                                                                person_index: 3),
@@ -66,7 +66,7 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
                                                                                              person_index: 3),
       Form::Lettings::Pages::PersonUnderRetirementValueCheck.new("age_3_under_retirement_value_check", nil, self, person_index: 3),
       Form::Lettings::Pages::PersonOverRetirementValueCheck.new("age_3_over_retirement_value_check", nil, self, person_index: 3),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_3_partner_under_16_value_check", nil, self, person_index: 3) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_3_partner_under_16_value_check", nil, self, person_index: 3) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::PersonGenderIdentity.new(nil, nil, self, person_index: 3),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonValueCheck.new(nil, nil, self, person_index: 3),
       Form::Lettings::Pages::FemalesInSoftAgeRangeInPregnantHouseholdPersonValueCheck.new(nil, nil, self,
@@ -79,8 +79,8 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
       Form::Lettings::Pages::PersonKnown.new(nil, nil, self, person_index: 4),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 4) if form.start_year_2026_or_later?),
       relationship_question(person_index: 4),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_4_partner_under_16_value_check", nil, self, person_index: 4) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
-      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_4_multiple_partners_value_check", nil, self, person_index: 4) if form.start_year_2024_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_4_partner_under_16_value_check", nil, self, person_index: 4) unless form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_4_multiple_partners_value_check", nil, self, person_index: 4) unless form.start_year_2026_or_later?),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 4) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonAgeValueCheck.new(nil, nil, self,
                                                                                person_index: 4),
@@ -88,7 +88,7 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
                                                                                              person_index: 4),
       Form::Lettings::Pages::PersonUnderRetirementValueCheck.new("age_4_under_retirement_value_check", nil, self, person_index: 4),
       Form::Lettings::Pages::PersonOverRetirementValueCheck.new("age_4_over_retirement_value_check", nil, self, person_index: 4),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_4_partner_under_16_value_check", nil, self, person_index: 4) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_4_partner_under_16_value_check", nil, self, person_index: 4) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::PersonGenderIdentity.new(nil, nil, self, person_index: 4),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonValueCheck.new(nil, nil, self, person_index: 4),
       Form::Lettings::Pages::FemalesInSoftAgeRangeInPregnantHouseholdPersonValueCheck.new(nil, nil, self,
@@ -101,8 +101,8 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
       Form::Lettings::Pages::PersonKnown.new(nil, nil, self, person_index: 5),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 5) if form.start_year_2026_or_later?),
       relationship_question(person_index: 5),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_5_partner_under_16_value_check", nil, self, person_index: 5) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
-      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_5_multiple_partners_value_check", nil, self, person_index: 5) if form.start_year_2024_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_5_partner_under_16_value_check", nil, self, person_index: 5) unless form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_5_multiple_partners_value_check", nil, self, person_index: 5) unless form.start_year_2026_or_later?),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 5) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonAgeValueCheck.new(nil, nil, self,
                                                                                person_index: 5),
@@ -110,7 +110,7 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
                                                                                              person_index: 5),
       Form::Lettings::Pages::PersonUnderRetirementValueCheck.new("age_5_under_retirement_value_check", nil, self, person_index: 5),
       Form::Lettings::Pages::PersonOverRetirementValueCheck.new("age_5_over_retirement_value_check", nil, self, person_index: 5),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_5_partner_under_16_value_check", nil, self, person_index: 5) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_5_partner_under_16_value_check", nil, self, person_index: 5) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::PersonGenderIdentity.new(nil, nil, self, person_index: 5),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonValueCheck.new(nil, nil, self, person_index: 5),
       Form::Lettings::Pages::FemalesInSoftAgeRangeInPregnantHouseholdPersonValueCheck.new(nil, nil, self,
@@ -123,8 +123,8 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
       Form::Lettings::Pages::PersonKnown.new(nil, nil, self, person_index: 6),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 6) if form.start_year_2026_or_later?),
       relationship_question(person_index: 6),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_6_partner_under_16_value_check", nil, self, person_index: 6) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
-      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_6_multiple_partners_value_check", nil, self, person_index: 6) if form.start_year_2024_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_6_partner_under_16_value_check", nil, self, person_index: 6) unless form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_6_multiple_partners_value_check", nil, self, person_index: 6) unless form.start_year_2026_or_later?),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 6) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonAgeValueCheck.new(nil, nil, self,
                                                                                person_index: 6),
@@ -132,7 +132,7 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
                                                                                              person_index: 6),
       Form::Lettings::Pages::PersonUnderRetirementValueCheck.new("age_6_under_retirement_value_check", nil, self, person_index: 6),
       Form::Lettings::Pages::PersonOverRetirementValueCheck.new("age_6_over_retirement_value_check", nil, self, person_index: 6),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_6_partner_under_16_value_check", nil, self, person_index: 6) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_6_partner_under_16_value_check", nil, self, person_index: 6) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::PersonGenderIdentity.new(nil, nil, self, person_index: 6),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonValueCheck.new(nil, nil, self, person_index: 6),
       Form::Lettings::Pages::FemalesInSoftAgeRangeInPregnantHouseholdPersonValueCheck.new(nil, nil, self,
@@ -145,8 +145,8 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
       Form::Lettings::Pages::PersonKnown.new(nil, nil, self, person_index: 7),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 7) if form.start_year_2026_or_later?),
       relationship_question(person_index: 7),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_7_partner_under_16_value_check", nil, self, person_index: 7) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
-      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_7_multiple_partners_value_check", nil, self, person_index: 7) if form.start_year_2024_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_7_partner_under_16_value_check", nil, self, person_index: 7) unless form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_7_multiple_partners_value_check", nil, self, person_index: 7) unless form.start_year_2026_or_later?),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 7) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonAgeValueCheck.new(nil, nil, self,
                                                                                person_index: 7),
@@ -154,7 +154,7 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
                                                                                              person_index: 7),
       Form::Lettings::Pages::PersonUnderRetirementValueCheck.new("age_7_under_retirement_value_check", nil, self, person_index: 7),
       Form::Lettings::Pages::PersonOverRetirementValueCheck.new("age_7_over_retirement_value_check", nil, self, person_index: 7),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_7_partner_under_16_value_check", nil, self, person_index: 7) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_7_partner_under_16_value_check", nil, self, person_index: 7) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::PersonGenderIdentity.new(nil, nil, self, person_index: 7),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonValueCheck.new(nil, nil, self, person_index: 7),
       Form::Lettings::Pages::FemalesInSoftAgeRangeInPregnantHouseholdPersonValueCheck.new(nil, nil, self,
@@ -167,8 +167,8 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
       Form::Lettings::Pages::PersonKnown.new(nil, nil, self, person_index: 8),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 8) if form.start_year_2026_or_later?),
       relationship_question(person_index: 8),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_8_partner_under_16_value_check", nil, self, person_index: 8) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
-      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_8_multiple_partners_value_check", nil, self, person_index: 8) if form.start_year_2024_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("relationship_8_partner_under_16_value_check", nil, self, person_index: 8) unless form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::MultiplePartnersValueCheck.new("relationship_8_multiple_partners_value_check", nil, self, person_index: 8) unless form.start_year_2026_or_later?),
       (Form::Lettings::Pages::PersonAge.new(nil, nil, self, person_index: 8) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonAgeValueCheck.new(nil, nil, self,
                                                                                person_index: 8),
@@ -176,7 +176,7 @@ class Form::Lettings::Subsections::HouseholdCharacteristics < ::Form::Subsection
                                                                                              person_index: 8),
       Form::Lettings::Pages::PersonUnderRetirementValueCheck.new("age_8_under_retirement_value_check", nil, self, person_index: 8),
       Form::Lettings::Pages::PersonOverRetirementValueCheck.new("age_8_over_retirement_value_check", nil, self, person_index: 8),
-      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_8_partner_under_16_value_check", nil, self, person_index: 8) if form.start_year_2024_or_later? && !form.start_year_2026_or_later?),
+      (Form::Lettings::Pages::PartnerUnder16ValueCheck.new("age_8_partner_under_16_value_check", nil, self, person_index: 8) unless form.start_year_2026_or_later?),
       Form::Lettings::Pages::PersonGenderIdentity.new(nil, nil, self, person_index: 8),
       Form::Lettings::Pages::NoFemalesPregnantHouseholdPersonValueCheck.new(nil, nil, self, person_index: 8),
       Form::Lettings::Pages::FemalesInSoftAgeRangeInPregnantHouseholdPersonValueCheck.new(nil, nil, self,
diff --git a/app/models/lettings_log.rb b/app/models/lettings_log.rb
index f2230c4e5..cdd52799b 100644
--- a/app/models/lettings_log.rb
+++ b/app/models/lettings_log.rb
@@ -876,6 +876,10 @@ private
     end
   end
 
+  def partner_numbers
+    (2..8).select { |i| public_send("relat#{i}") == "P" }
+  end
+
   def age_refused?
     [age1_known, age2_known, age3_known, age4_known, age5_known, age6_known, age7_known, age8_known].any?(1)
   end
diff --git a/spec/factories/lettings_log.rb b/spec/factories/lettings_log.rb
index 62cc94a14..d65133894 100644
--- a/spec/factories/lettings_log.rb
+++ b/spec/factories/lettings_log.rb
@@ -131,7 +131,7 @@ FactoryBot.define do
       rp_hardship { 0 }
       rp_dontknow { 0 }
       tenancyother { nil }
-      net_income_value_check { nil }
+      net_income_value_check { 1 }
       void_date_value_check { 1 }
       major_repairs_date_value_check { 1 }
       net_income_known { 0 }
diff --git a/spec/models/form/lettings/pages/person_lead_partner_spec.rb b/spec/models/form/lettings/pages/person_lead_partner_spec.rb
index 55ee97e1d..adff653da 100644
--- a/spec/models/form/lettings/pages/person_lead_partner_spec.rb
+++ b/spec/models/form/lettings/pages/person_lead_partner_spec.rb
@@ -96,8 +96,13 @@ RSpec.describe Form::Lettings::Pages::PersonLeadPartner, type: :model do
                 "operator" => ">=",
                 "operand" => 16,
               },
+              "relat2" => { "operator" => "!=", "operand" => "P" },
+            },
+            {
+              "details_known_3" => 0,
+              "age3" => nil,
+              "relat2" => { "operator" => "!=", "operand" => "P" },
             },
-            { "details_known_3" => 0, "age3" => nil },
           ],
         )
       end
diff --git a/spec/models/form/lettings/questions/person_partner_spec.rb b/spec/models/form/lettings/questions/person_partner_spec.rb
index 2106bc36f..1770526dc 100644
--- a/spec/models/form/lettings/questions/person_partner_spec.rb
+++ b/spec/models/form/lettings/questions/person_partner_spec.rb
@@ -105,5 +105,9 @@ RSpec.describe Form::Lettings::Questions::PersonPartner, type: :model do
     it "has the correct check_answers_card_number" do
       expect(question.check_answers_card_number).to eq(3)
     end
+
+    it "has the correct hidden_in_check_answers" do
+      expect(question.hidden_in_check_answers).to eq("relat2" => "P", "relat3" => "X")
+    end
   end
 end
diff --git a/spec/models/form/lettings/subsections/household_characteristics_spec.rb b/spec/models/form/lettings/subsections/household_characteristics_spec.rb
index 3269514d2..35c62ad84 100644
--- a/spec/models/form/lettings/subsections/household_characteristics_spec.rb
+++ b/spec/models/form/lettings/subsections/household_characteristics_spec.rb
@@ -364,7 +364,6 @@ RSpec.describe Form::Lettings::Subsections::HouseholdCharacteristics, type: :mod
           person_2_known
           person_2_age
           person_2_lead_partner
-          relationship_2_multiple_partners_value_check
           no_females_pregnant_household_person_2_age_value_check
           females_in_soft_age_range_in_pregnant_household_person_2_age_value_check
           age_2_under_retirement_value_check
@@ -380,7 +379,6 @@ RSpec.describe Form::Lettings::Subsections::HouseholdCharacteristics, type: :mod
           person_3_known
           person_3_age
           person_3_lead_partner
-          relationship_3_multiple_partners_value_check
           no_females_pregnant_household_person_3_age_value_check
           females_in_soft_age_range_in_pregnant_household_person_3_age_value_check
           age_3_under_retirement_value_check
@@ -396,7 +394,6 @@ RSpec.describe Form::Lettings::Subsections::HouseholdCharacteristics, type: :mod
           person_4_known
           person_4_age
           person_4_lead_partner
-          relationship_4_multiple_partners_value_check
           no_females_pregnant_household_person_4_age_value_check
           females_in_soft_age_range_in_pregnant_household_person_4_age_value_check
           age_4_under_retirement_value_check
@@ -412,7 +409,6 @@ RSpec.describe Form::Lettings::Subsections::HouseholdCharacteristics, type: :mod
           person_5_known
           person_5_age
           person_5_lead_partner
-          relationship_5_multiple_partners_value_check
           no_females_pregnant_household_person_5_age_value_check
           females_in_soft_age_range_in_pregnant_household_person_5_age_value_check
           age_5_under_retirement_value_check
@@ -428,7 +424,6 @@ RSpec.describe Form::Lettings::Subsections::HouseholdCharacteristics, type: :mod
           person_6_known
           person_6_age
           person_6_lead_partner
-          relationship_6_multiple_partners_value_check
           no_females_pregnant_household_person_6_age_value_check
           females_in_soft_age_range_in_pregnant_household_person_6_age_value_check
           age_6_under_retirement_value_check
@@ -444,7 +439,6 @@ RSpec.describe Form::Lettings::Subsections::HouseholdCharacteristics, type: :mod
           person_7_known
           person_7_age
           person_7_lead_partner
-          relationship_7_multiple_partners_value_check
           no_females_pregnant_household_person_7_age_value_check
           females_in_soft_age_range_in_pregnant_household_person_7_age_value_check
           age_7_under_retirement_value_check
@@ -460,7 +454,6 @@ RSpec.describe Form::Lettings::Subsections::HouseholdCharacteristics, type: :mod
           person_8_known
           person_8_age
           person_8_lead_partner
-          relationship_8_multiple_partners_value_check
           no_females_pregnant_household_person_8_age_value_check
           females_in_soft_age_range_in_pregnant_household_person_8_age_value_check
           age_8_under_retirement_value_check
diff --git a/spec/models/lettings_log_derived_fields_spec.rb b/spec/models/lettings_log_derived_fields_spec.rb
index 9cb6cf7d1..9c9762e8e 100644
--- a/spec/models/lettings_log_derived_fields_spec.rb
+++ b/spec/models/lettings_log_derived_fields_spec.rb
@@ -1588,4 +1588,156 @@ RSpec.describe LettingsLog, type: :model do
       end
     end
   end
+
+  describe "#infer_at_most_one_relationship!" do
+    context "when 2025", metadata: { year: 25 } do
+      before do
+        Timecop.freeze(Time.zone.local(2025, 5, 10))
+        Singleton.__init__(FormHandler)
+      end
+
+      after do
+        Timecop.return
+        Singleton.__init__(FormHandler)
+      end
+
+      context "when there are no existing relationships" do
+        let(:log) { create(:lettings_log, :completed, relat2: "X", relat3: "X", relat4: "R") }
+
+        it "does not infer no to any relationship answers when a new relationship is added" do
+          log.relat2 = "P"
+          expect { log.set_derived_fields! }.to not_change(log, :relat3)
+          expect { log.set_derived_fields! }.to not_change(log, :relat4)
+          expect { log.set_derived_fields! }.to not_change(log, :relat5)
+        end
+      end
+
+      context "when there is an existing relationship" do
+        let(:log) { create(:lettings_log, :completed, relat2: "X", relat3: "P", relat4: "R", relat5: "X") }
+
+        it "does not infer no to any relationship answers when a new relationship is added" do
+          log.relat2 = "P"
+          expect { log.set_derived_fields! }.to not_change(log, :relat3)
+          expect { log.set_derived_fields! }.to not_change(log, :relat4)
+          expect { log.set_derived_fields! }.to not_change(log, :relat5)
+          expect { log.set_derived_fields! }.to not_change(log, :relat6)
+        end
+      end
+
+      context "when more than one relationship is set" do
+        let(:log) { create(:lettings_log, :completed, relat2: "X", relat3: "X", relat4: "R", relat5: "X") }
+
+        before do
+          log.relat3 = "P"
+          log.relat2 = "P"
+        end
+
+        it "does not infer no to any relationship answers" do
+          expect { log.set_derived_fields! }.to not_change(log, :relat2)
+          expect { log.set_derived_fields! }.to not_change(log, :relat3)
+          expect { log.set_derived_fields! }.to not_change(log, :relat4)
+          expect { log.set_derived_fields! }.to not_change(log, :relat5)
+          expect { log.set_derived_fields! }.to not_change(log, :relat6)
+        end
+      end
+    end
+
+    context "when 2026", metadata: { year: 26 } do
+      before do
+        Timecop.freeze(Time.zone.local(2026, 5, 10))
+        Singleton.__init__(FormHandler)
+      end
+
+      after do
+        Timecop.return
+        Singleton.__init__(FormHandler)
+      end
+
+      context "when there are no existing relationships" do
+        let(:log) { create(:lettings_log, :completed, relat2: "X", relat3: "X", relat4: "R") }
+
+        context "and a new relationship is added" do
+          before do
+            log.relat2 = "P"
+          end
+
+          it "infers no to unanswered questions" do
+            expect { log.set_derived_fields! }.to change(log, :relat5).to "X"
+          end
+
+          it "does not change relationship answers of no or prefer not to say" do
+            expect { log.set_derived_fields! }.to not_change(log, :relat3)
+            expect { log.set_derived_fields! }.to not_change(log, :relat4)
+          end
+        end
+
+        it "does not change other relationship values if no is changed to prefer not to say" do
+          log.relat2 = "R"
+          expect { log.set_derived_fields! }.to not_change(log, :relat3)
+          expect { log.set_derived_fields! }.to not_change(log, :relat4)
+        end
+
+        it "does not change other relationship values if prefer not to say is changed to no" do
+          log.relat4 = "X"
+          expect { log.set_derived_fields! }.to not_change(log, :relat2)
+          expect { log.set_derived_fields! }.to not_change(log, :relat3)
+        end
+      end
+
+      context "when there is an existing relationship" do
+        let(:log) { create(:lettings_log, :completed, relat2: "X", relat3: "P", relat4: "R", relat5: "X") }
+
+        context "and a new relationship is added" do
+          before do
+            log.relat6 = nil
+            log.relat2 = "P"
+          end
+
+          it "infers no to the previous relationship" do
+            expect { log.set_derived_fields! }.to change(log, :relat3).to "X"
+          end
+
+          it "infers no to unanswered questions" do
+            expect { log.set_derived_fields! }.to change(log, :relat6).to "X"
+          end
+
+          it "does not change relationship answers of no or prefer not to say" do
+            expect { log.set_derived_fields! }.to not_change(log, :relat5)
+            expect { log.set_derived_fields! }.to not_change(log, :relat4)
+          end
+        end
+
+        it "does not change other relationship values if the partner is removed" do
+          log.relat3 = "X"
+          expect { log.set_derived_fields! }.to not_change(log, :relat2)
+          expect { log.set_derived_fields! }.to not_change(log, :relat4)
+          expect { log.set_derived_fields! }.to not_change(log, :relat5)
+          expect { log.set_derived_fields! }.to not_change(log, :relat6)
+        end
+      end
+
+      context "when more than one relationship is set" do
+        let(:log) { create(:lettings_log, :completed, hhmemb: 6, relat2: "X", relat3: "X", relat4: "R", relat5: "X") }
+
+        before do
+          log.relat3 = "P"
+          log.relat2 = "P"
+        end
+
+        it "keeps the lower numbered relationship and infers the higher numbered one to false" do
+          expect { log.set_derived_fields! }.to change(log, :relat3).to "X"
+          expect { log.set_derived_fields! }.to not_change(log, :relat2)
+        end
+
+        it "infers no to unanswered questions" do
+          expect { log.set_derived_fields! }.to change(log, :relat6).to "X"
+        end
+
+        it "does not change relationship answers of no or prefer not to say" do
+          expect { log.set_derived_fields! }.to not_change(log, :relat5)
+          expect { log.set_derived_fields! }.to not_change(log, :relat4)
+        end
+      end
+    end
+  end
 end