diff --git a/datatypes/epc/domain/tests/test_from_rdsap_schema.py b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
index 8e88bb77..3ff99b4e 100644
--- a/datatypes/epc/domain/tests/test_from_rdsap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
@@ -1553,3 +1553,55 @@ class TestRdSap18_0ReducedFieldSynthesis:
         # Assert
         total_area = sum(w.window_width * w.window_height for w in result.sap_windows)
         assert total_area == pytest.approx(0.148 * tfa * 1.25)
+
+    def test_synthesised_glazing_type_routed_through_cascade(self) -> None:
+        # Arrange — ADR-0028: 18.0 multiple_glazing_type shares 20.0.0's code
+        # space (verified vs epc_codes.csv), so route it through the verified
+        # cascade — code 1 ("DG pre-2002") must remap to 2, not be read as single.
+        # A windowless cert lodging multiple_glazing_type=1.
+        corpus = _load_18_0_corpus()
+        if not corpus:
+            pytest.skip("no RdSAP-Schema-18.0 corpus harvested")
+        cert = next(
+            (
+                c
+                for c in corpus
+                if not c.get("sap_windows") and c.get("multiple_glazing_type") == 1
+            ),
+            None,
+        )
+        if cert is None:
+            pytest.skip("no corpus cert with multiple_glazing_type=1")
+
+        # Act
+        result = EpcPropertyDataMapper.from_api_response(cert)
+
+        # Assert — cascade remaps 1 ("DG pre-2002") -> 2 (double), not raw 1.
+        assert all(w.glazing_type == 2 for w in result.sap_windows)
+
+    def test_synthesised_glazing_type_handles_not_defined_code(self) -> None:
+        # Arrange — ADR-0028: 69/1000 18.0 certs lodge multiple_glazing_type "ND"
+        # (Not Defined), a string the int-keyed cascade cannot map. The
+        # synthesised window must carry a valid INTEGER glazing_type (treated as
+        # DG-modal, matching the calculator's _G_LIGHT_DEFAULT), never the raw
+        # "ND" string on an int field. A windowless cert lodging "ND".
+        corpus = _load_18_0_corpus()
+        if not corpus:
+            pytest.skip("no RdSAP-Schema-18.0 corpus harvested")
+        cert = next(
+            (
+                c
+                for c in corpus
+                if not c.get("sap_windows") and c.get("multiple_glazing_type") == "ND"
+            ),
+            None,
+        )
+        if cert is None:
+            pytest.skip("no windowless corpus cert with multiple_glazing_type ND")
+
+        # Act
+        result = EpcPropertyDataMapper.from_api_response(cert)
+
+        # Assert — every synthesised window has an int glazing_type, not "ND".
+        assert result.sap_windows
+        assert all(isinstance(w.glazing_type, int) for w in result.sap_windows)