diff --git a/app/models/case_log.rb b/app/models/case_log.rb
index 4a3af3c75..1d504ac19 100644
--- a/app/models/case_log.rb
+++ b/app/models/case_log.rb
@@ -50,7 +50,7 @@ class CaseLog < ApplicationRecord
                          }
 
   AUTOGENERATED_FIELDS = %w[id status created_at updated_at discarded_at].freeze
-  OPTIONAL_FIELDS = %w[postcode_known previous_la_known first_time_property_let_as_social_housing tenant_code propcode].freeze
+  OPTIONAL_FIELDS = %w[first_time_property_let_as_social_housing tenant_code propcode].freeze
   RENT_TYPE_MAPPING = { 0 => 1, 1 => 2, 2 => 2, 3 => 3, 4 => 3, 5 => 3 }.freeze
   RENT_TYPE_MAPPING_LABELS = { 1 => "Social Rent", 2 => "Affordable Rent", 3 => "Intermediate Rent" }.freeze
   HAS_BENEFITS_OPTIONS = [1, 6, 8, 7].freeze
@@ -419,7 +419,8 @@ private
   end
 
   def dynamically_not_required
-    (form.invalidated_questions(self) + form.readonly_questions).map(&:id).uniq
+    previous_la_known_field = postcode_known? ? %w[previous_la_known] : []
+    ((form.invalidated_questions(self) + form.readonly_questions).map(&:id) + previous_la_known_field).uniq
   end
 
   def set_derived_fields!
diff --git a/spec/factories/case_log.rb b/spec/factories/case_log.rb
index 2b1f780ac..11a38de09 100644
--- a/spec/factories/case_log.rb
+++ b/spec/factories/case_log.rb
@@ -103,6 +103,7 @@ FactoryBot.define do
       tenancyother { nil }
       net_income_value_check { nil }
       net_income_known { 1 }
+      previous_la_known { 1 }
       property_owner_organisation { "Test" }
       property_manager_organisation { "Test" }
       renewal { 0 }
diff --git a/spec/fixtures/exports/case_logs.xml b/spec/fixtures/exports/case_logs.xml
index 1d712eed8..4c91a1271 100644
--- a/spec/fixtures/exports/case_logs.xml
+++ b/spec/fixtures/exports/case_logs.xml
@@ -132,7 +132,7 @@
     <chcharge/>
     <declaration>1</declaration>
     <previous_postcode_known>1</previous_postcode_known>
-    <previous_la_known/>
+    <previous_la_known>1</previous_la_known>
     <is_previous_la_inferred>false</is_previous_la_inferred>
     <ethnic_other/>
     <letting_allocation_unknown/>