diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 90428bb2e..e3cdde0be 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2420,7 +2420,7 @@ class EpcPropertyDataMapper:
             sap_version=schema.sap_version,
             dwelling_type=schema.dwelling_type,
             property_type=str(schema.property_type),
-            built_form=str(schema.built_form),
+            built_form=None if schema.built_form is None else str(schema.built_form),
             address_line_1=schema.address_line_1,
             address_line_2=schema.address_line_2,
             postcode=schema.postcode,
diff --git a/datatypes/epc/schema/rdsap_schema_21_0_1.py b/datatypes/epc/schema/rdsap_schema_21_0_1.py
index 4b814d7fb..c0d219568 100644
--- a/datatypes/epc/schema/rdsap_schema_21_0_1.py
+++ b/datatypes/epc/schema/rdsap_schema_21_0_1.py
@@ -460,7 +460,6 @@ class RdSapSchema21_0_1:
     postcode: str
     hot_water: EnergyElement
     post_town: str
-    built_form: int
     door_count: int
     region_code: int
     report_type: int
@@ -512,9 +511,16 @@ class RdSapSchema21_0_1:
     renewable_heat_incentive: RenewableHeatIncentive
     draughtproofed_door_count: int
     energy_consumption_current: int
-    # kw_only on just this field: the class isn't kw_only overall, and several
-    # required fields (calculation_software_version etc.) already follow it.
+    # kw_only on just these fields: the class isn't kw_only overall, and several
+    # required fields (calculation_software_version etc.) already follow them.
     has_fixed_air_conditioning: Optional[str] = field(default=None, kw_only=True)
+    # Some real-API certs omit `built_form` entirely (e.g. cert
+    # 2461-5385-9264-6821-5357, property_id=750232, portfolio 824) rather than
+    # sending a code — `from_dict` otherwise hard-fails the whole cert. The
+    # mapper/SAP cascade (`_api_sheltered_sides`) already treats a non-int
+    # built_form as "no lodging" and falls back to the documented default, so
+    # None here is an already-safe degradation, not a new failure mode.
+    built_form: Optional[int] = field(default=None, kw_only=True)
     calculation_software_version: str
     energy_consumption_potential: int
     environmental_impact_current: int
diff --git a/datatypes/epc/schema/tests/test_schema_loading.py b/datatypes/epc/schema/tests/test_schema_loading.py
index 3f2759bf8..ea48b092a 100644
--- a/datatypes/epc/schema/tests/test_schema_loading.py
+++ b/datatypes/epc/schema/tests/test_schema_loading.py
@@ -400,3 +400,17 @@ class TestRdSapSchema21_0_1AgainstRealApiCert:
         assert epc.schema_type == "RdSAP-Schema-21.0.1"
         assert epc.sap_heating is not None
         assert len(epc.sap_windows) > 0
+
+    def test_cert_with_no_built_form_key_parses_as_none(self) -> None:
+        # Arrange — some real gov-API certs omit `built_form` entirely (not
+        # null: absent). property_id=750232 (portfolio 824) hit exactly this
+        # on cert 2461-5385-9264-6821-5357 and failed to parse at all.
+        real_doc = load("21_0_1_real.json")
+        del real_doc["built_form"]
+
+        # Act
+        epc = from_dict(RdSapSchema21_0_1, real_doc)
+
+        # Assert — parsing no longer hard-fails; the field degrades to None,
+        # same as the mapper/SAP-cascade already do for a non-int built_form.
+        assert epc.built_form is None