diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 1985db439..957573249 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -5990,6 +5990,11 @@ def _map_main_building_part(
     return SapBuildingPart(
         identifier=BuildingPartIdentifier.MAIN,
         construction_age_band=_extract_age_band(main.age_range),
+        # PasHub surveys never lodge a basement wall; pin the flag so a
+        # "System Build" wall (code 6, which doubles as the gov-API basement
+        # sentinel) is not silently routed through the basement wall/floor
+        # cascade (issue #1590 bug 7).
+        wall_is_basement=False,
         wall_construction=_pashub_wall_construction_int(
             main.walls_construction_type
         ),
@@ -6021,6 +6026,11 @@ def _map_extension_building_part(
     return SapBuildingPart(
         identifier=BuildingPartIdentifier.extension(ext_c.id),
         construction_age_band=_extract_age_band(ext_c.age_range),
+        # PasHub surveys never lodge a basement wall; pin the flag so a
+        # "System Build" wall (code 6, which doubles as the gov-API basement
+        # sentinel) is not silently routed through the basement wall/floor
+        # cascade (issue #1590 bug 7).
+        wall_is_basement=False,
         wall_construction=_pashub_wall_construction_int(
             ext_c.walls_construction_type
         ),
diff --git a/datatypes/epc/domain/tests/test_from_site_notes.py b/datatypes/epc/domain/tests/test_from_site_notes.py
index 151fbbf1e..42f8e62bb 100644
--- a/datatypes/epc/domain/tests/test_from_site_notes.py
+++ b/datatypes/epc/domain/tests/test_from_site_notes.py
@@ -1145,6 +1145,7 @@ class TestFromSiteNotesExample1:
                 SapBuildingPart(
                     identifier=BuildingPartIdentifier.MAIN,
                     construction_age_band="I",
+                    wall_is_basement=False,  # PasHub never lodges a basement wall
                     wall_construction=4,  # "Cavity" → SAP10 code 4 (WALL_CAVITY)
                     wall_insulation_type=4,  # "As built" → SAP10 code 4 (as built/assumed)
                     wall_thickness_measured=True,