diff --git a/tests/domain/modelling/test_wall_recommendation.py b/tests/domain/modelling/test_wall_recommendation.py
index 8fe2620d4..993d7a856 100644
--- a/tests/domain/modelling/test_wall_recommendation.py
+++ b/tests/domain/modelling/test_wall_recommendation.py
@@ -90,6 +90,24 @@ def test_non_cavity_main_wall_yields_no_recommendation() -> None:
     assert recommendation is None
 
 
+def test_at_regs_age_band_cavity_yields_no_cavity_fill_recommendation() -> None:
+    # Arrange — a cavity wall lodged as-built at construction age band L
+    # (2012-2022): the as-built cascade already assigns the at-regs U-value, so
+    # filling the cavity cannot lower it. The Wall U-Value Gate withholds the
+    # Option (ADR-0051); the legacy insulation-state trigger alone would offer
+    # a paid no-op.
+    baseline: EpcPropertyData = build_epc()  # MAIN: cavity (4), uninsulated (4)
+    _part(baseline, BuildingPartIdentifier.MAIN).construction_age_band = "L"
+
+    # Act
+    recommendation: Recommendation | None = recommend_cavity_wall(
+        baseline, _StubProducts()
+    )
+
+    # Assert
+    assert recommendation is None
+
+
 def test_at_regs_age_band_wall_yields_no_solid_wall_recommendation() -> None:
     # Arrange — a timber-frame wall lodged as-built at construction age band L
     # (2012-2022). RdSAP's cascade assigns it the at-regs U-value (0.28), and