diff --git a/datatypes/epc/domain/tests/test_from_site_notes.py b/datatypes/epc/domain/tests/test_from_site_notes.py
index de38e214a..78eb89da0 100644
--- a/datatypes/epc/domain/tests/test_from_site_notes.py
+++ b/datatypes/epc/domain/tests/test_from_site_notes.py
@@ -240,6 +240,80 @@ class TestRoomInRoofMapping:
         assert by_kind["flat_ceiling"] == 100
         assert by_kind["gable_wall"] is None
 
+    # A Simplified-approach lodgement ("Roof room type: RR type 2", details
+    # NOT known): gables + common walls only, no slopes/flat ceilings.
+    # Verbatim from fixture 499538003156 (78 North Road).
+    _RIR_SIMPLIFIED = {
+        "age_range": "C: 1930 - 1949",
+        "floor_area_m2": 25.43,
+        "roof_room_type": "RR type 2 (With the Accessible areas of continuous common walls)",
+        "gables": [
+            {"length_m": 5.82, "height_m": 2.72, "gable_type": "Party"},
+            {"length_m": 5.82, "height_m": 2.72, "gable_type": "Party"},
+        ],
+        "common_walls": [
+            {"length_m": 4.37, "height_m": 2.34},
+            {"length_m": 4.37, "height_m": 2.34},
+        ],
+    }
+
+    def test_simplified_common_walls_bill_as_common_wall(self) -> None:
+        """On the Simplified approach (a "Roof room type" is lodged), a
+        "common wall" is a §3.9.2 Type-2 concept billed as an external wall
+        at the main-wall U — SAP kind `common_wall`, spec area L × (0.25 + H)
+        (RdSAP 10 §3.9.2 / heat_transmission's Simplified-BP area rule) — NOT
+        a Detailed §3.10 `stud_wall` roof surface, which would drop to the
+        Table 18 col(4) "all elements uninsulated" default (2.30 W/m²K) over
+        ~20 m² on the no-loft-access North Road pair (deals
+        499538003156/499627236560, over-rate ~5 SAP before this fix)."""
+        # Arrange
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["roof_space"]["main_building"]["rooms_in_roof"] = True
+        data["roof_space"]["main_building"]["room_in_roof"] = self._RIR_SIMPLIFIED
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+
+        # Act
+        rir = EpcPropertyDataMapper.from_site_notes(survey).sap_building_parts[
+            0
+        ].sap_room_in_roof
+
+        # Assert
+        assert rir is not None
+        surfaces = rir.detailed_surfaces
+        assert surfaces is not None
+        cw_area = round(4.37 * (0.25 + 2.34), 2)  # L × (0.25 + H)
+        kinds_areas = [(s.kind, round(s.area_m2, 2)) for s in surfaces]
+        assert kinds_areas == [
+            ("gable_wall", round(5.82 * 2.72, 2)),
+            ("gable_wall", round(5.82 * 2.72, 2)),
+            ("common_wall", cw_area),
+            ("common_wall", cw_area),
+        ]
+
+    def test_detailed_common_walls_stay_stud_wall(self) -> None:
+        """Regression: a Detailed §3.10 lodgement (no "Roof room type", per-
+        surface slopes lodged) keeps common walls as `stud_wall` — pinned
+        alongside `test_room_in_roof_reaches_building_part`."""
+        # Arrange — the Detailed fixture has no roof_room_type.
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["roof_space"]["main_building"]["rooms_in_roof"] = True
+        data["roof_space"]["main_building"]["room_in_roof"] = self._RIR
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+
+        # Act
+        rir = EpcPropertyDataMapper.from_site_notes(survey).sap_building_parts[
+            0
+        ].sap_room_in_roof
+
+        # Assert
+        assert rir is not None
+        assert rir.detailed_surfaces is not None
+        assert {s.kind for s in rir.detailed_surfaces if "wall" in s.kind} == {
+            "gable_wall_external",
+            "gable_wall",
+            "stud_wall",
+        }
+
     def test_no_rooms_in_roof_stays_absent(self) -> None:
         # Arrange — the fixture as lodged (rooms_in_roof=false).
         survey = from_dict(