diff --git a/infrastructure/epc_client/tests/test_mapper_corpus.py b/infrastructure/epc_client/tests/test_mapper_corpus.py
index 720ae0ee..a674b5c6 100644
--- a/infrastructure/epc_client/tests/test_mapper_corpus.py
+++ b/infrastructure/epc_client/tests/test_mapper_corpus.py
@@ -80,3 +80,22 @@ def test_rdsap_19_0_maps() -> None:
 
     # Assert
     assert isinstance(result, EpcPropertyData)
+
+
+def test_rdsap_19_0_synthesises_windows_for_windowless_cert() -> None:
+    """ADR-0028 Reduced-Field Synthesis: a reduced 19.0 cert lodges no
+    per-window geometry, only a glazed_area band — the mapper synthesises a
+    4-way N/E/S/W split rather than leaving the dwelling windowless."""
+    # Arrange — a band-1 cert with no lodged sap_windows array
+    certs = _load("RdSAP-Schema-19.0")
+    if not certs:
+        pytest.skip("no RdSAP-Schema-19.0 corpus harvested")
+    cert = next(
+        c for c in certs if c.get("glazed_area") == 1 and not c.get("sap_windows")
+    )
+
+    # Act
+    result = EpcPropertyDataMapper.from_api_response(cert)
+
+    # Assert — N, E, S, W (SAP orientation codes 1, 3, 5, 7)
+    assert [w.orientation for w in result.sap_windows] == [1, 3, 5, 7]