diff --git a/datatypes/epc/domain/tests/test_from_site_notes.py b/datatypes/epc/domain/tests/test_from_site_notes.py
index 844a17a03..151fbbf1e 100644
--- a/datatypes/epc/domain/tests/test_from_site_notes.py
+++ b/datatypes/epc/domain/tests/test_from_site_notes.py
@@ -122,6 +122,32 @@ class TestPhotovoltaicArrayMapping:
         assert result.sap_energy_source.photovoltaic_arrays is None
 
 
+class TestSystemBuildWallIsNotBasement:
+    """RdSAP wall-construction code 6 is overloaded: PasHub "System Build
+    (i.e Any Other)" maps to 6, but 6 doubles as the gov-API basement-wall
+    sentinel — with `wall_is_basement` left None, `main_wall_is_basement`
+    falls back to the code check and silently routes the dwelling through the
+    basement wall/floor cascade (issue #1590 bug 7). The mapper must pin the
+    flag False: a surveyed system-build wall is never a basement lodging.
+    """
+
+    def test_system_build_wall_does_not_route_to_basement(self) -> None:
+        # Arrange
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["building_construction"]["main_building"][
+            "walls_construction_type"
+        ] = "System Build (i.e Any Other)"
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+
+        # Act
+        result = EpcPropertyDataMapper.from_site_notes(survey)
+
+        # Assert — code 6, but explicitly NOT a basement.
+        part = result.sap_building_parts[0]
+        assert part.wall_construction == 6
+        assert part.main_wall_is_basement is False
+
+
 class TestCylinderSizeCoding:
     """The surveyed §Water-Heating "Cylinder Size" label must resolve to the
     SAP10 cylinder-size enum (`_cylinder_volume_l_from_code` int-or-nones a raw