diff --git a/tests/domain/epc/test_glazing_overlay.py b/tests/domain/epc/test_glazing_overlay.py
index 81c35e6ca..1c58fe590 100644
--- a/tests/domain/epc/test_glazing_overlay.py
+++ b/tests/domain/epc/test_glazing_overlay.py
@@ -48,6 +48,21 @@ def test_glazing_types_decode_to_their_sap_codes(
     assert simulation.glazing.glazing_type == code
 
 
+def test_secondary_glazing_overlays_its_glazing_code() -> None:
+    # Hyde-796 landlords assert "100% secondary glazing (sap 9.94)"; flattening it
+    # to Single applies U = 4.8 instead of secondary's 2.9 (Model#1416). The
+    # calculator already understands secondary glazing — SAP10 glazing_type code 5
+    # (`_GLAZING_CODE_TO_UWINDOW` in heat_transmission).
+
+    # Act
+    simulation = glazing_overlay_for("Secondary glazing", 0)
+
+    # Assert
+    assert simulation is not None
+    assert simulation.glazing is not None
+    assert simulation.glazing.glazing_type == 5
+
+
 @pytest.mark.parametrize("glazing_value", ["Unknown", ""])
 def test_unresolvable_glazing_produces_no_overlay(glazing_value: str) -> None:
     # Act