diff --git a/domain/sap10_calculator/worksheet/heat_transmission.py b/domain/sap10_calculator/worksheet/heat_transmission.py
index a658e668..a397fea1 100644
--- a/domain/sap10_calculator/worksheet/heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/heat_transmission.py
@@ -686,8 +686,26 @@ def heat_transmission_from_cert(
         # floor areas on each level. For a pitched roof with a sloping
         # ceiling, divide that area by cos(30°) — the worksheet enters
         # the inclined surface area, not the horizontal projection.
-        top_floor_area = geom["top_floor_area_m2"] if exposure.has_exposed_roof else 0.0
         roof_type = (part.roof_construction_type or "").lower()
+        # Per-BP roof exposure: an extension on a flat can have its own
+        # external roof even when the dwelling-level position says the
+        # primary building's roof is party (cohort cert 0036: ground-
+        # floor flat with single-storey Ext1 flat roof, worksheet (30)
+        # = 1.09 m² × U=2.30 = 2.51 W/K). The per-BP signal is the
+        # explicit `roof_construction_type` lodgement of "another
+        # dwelling above" — when present, suppress that BP's roof; when
+        # absent, the dwelling-level `exposure.has_exposed_roof` flag
+        # applies to the primary BP (i==0) and extensions (i>0) expose
+        # by default. Houses + bungalows pass through unchanged because
+        # their dwelling-level flag stays True.
+        part_roof_is_party = "another dwelling above" in roof_type
+        if part_roof_is_party:
+            part_has_exposed_roof = False
+        elif i == 0:
+            part_has_exposed_roof = exposure.has_exposed_roof
+        else:
+            part_has_exposed_roof = True
+        top_floor_area = geom["top_floor_area_m2"] if part_has_exposed_roof else 0.0
         if "sloping ceiling" in roof_type:
             top_floor_area = top_floor_area / _COS_30_DEG
         gross_roof_area = _round_half_up(top_floor_area, _AREA_ROUND_DP)
diff --git a/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py b/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py
index b3d43bf1..dbdeca5d 100644
--- a/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py
@@ -762,6 +762,64 @@ def test_ground_floor_flat_exposure_keeps_floor_drops_roof() -> None:
     assert ground.roof_w_per_k == 0.0
 
 
+def test_ground_floor_flat_extension_with_flat_roof_exposes_extension_roof_only() -> None:
+    """Per-BP roof exposure: an extension on a ground-floor flat can have
+    its own external (e.g. single-storey) roof even though the dwelling-
+    level position says the main building's roof is party. Cohort cert
+    0036-6325-1100-0063-1226: ground-floor flat, Main lodges roof type
+    "Another dwelling above" (party); Ext1 lodges roof type "Flat" with
+    its own external surface. Worksheet (30) sums Ext1's 1.09 m² × U=2.30
+    = 2.507 W/K; Main contributes 0 W/K. Without the per-BP signal the
+    dwelling-level `has_exposed_roof=False` zeroes both → -0.30 SAP
+    over-prediction.
+    """
+    # Arrange
+    main = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        construction_age_band="D",
+        wall_construction=4, wall_insulation_type=4,
+        party_wall_construction=1, roof_construction=4,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=60.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=30.0, floor=0,
+            ),
+        ],
+    )
+    main.roof_construction_type = "Another dwelling above"
+    ext1 = make_building_part(
+        identifier=BuildingPartIdentifier.EXTENSION_1,
+        construction_age_band="D",
+        wall_construction=4, wall_insulation_type=4,
+        party_wall_construction=1, roof_construction=4,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=1.09, room_height_m=2.0,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=3.0, floor=0,
+            ),
+        ],
+    )
+    ext1.roof_construction_type = "Flat"
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=61.09,
+        country_code="ENG",
+        sap_building_parts=[main, ext1],
+    )
+
+    # Act
+    result = heat_transmission_from_cert(
+        epc,
+        exposure=DwellingExposure(has_exposed_floor=True, has_exposed_roof=False),
+    )
+
+    # Assert — only Ext1's 1.09 m² flat roof contributes; Main's roof is
+    # party. Age D flat-roof default per Table 18 col (3) = 2.30 W/m²K.
+    expected_ext1_roof = 1.09 * 2.30
+    assert abs(result.roof_w_per_k - expected_ext1_roof) <= 0.01, (
+        f"got {result.roof_w_per_k:.4f}, want {expected_ext1_roof:.4f}"
+    )
+
+
 # ============================================================================
 # New §3 worksheet-line-mapped tests: alternative walls, effective window U,
 # and the (31)/(33) line-ref fields. Reference: SAP10.2 §3.2, RdSAP10 §1.4.2.