diff --git a/packages/domain/src/domain/sap/worksheet/dimensions.py b/packages/domain/src/domain/sap/worksheet/dimensions.py
index 28a7581c..56685c56 100644
--- a/packages/domain/src/domain/sap/worksheet/dimensions.py
+++ b/packages/domain/src/domain/sap/worksheet/dimensions.py
@@ -98,20 +98,22 @@ def dimensions_from_cert(epc: EpcPropertyData) -> Dimensions:
     top_area = 0.0
     gross_wall = 0.0
     party_wall = 0.0
-    total_storey_count = 0
+    # SAP §2 (9) "ns" is dwelling height (tallest part), NOT Σ across parts —
+    # the (10) additional-infiltration adjustment otherwise inflates by 0.1
+    # per spurious storey. Track per-part counts and take the max below.
+    part_storey_counts: list[int] = []
     for part in parts:
         ground = _part_ground_floor(part)
         top = _part_top_floor(part)
         if ground is None or top is None:
             continue
         part_height = _part_avg_storey_height_m(part)
-        part_storeys = _part_storey_count(part)
+        part_floor_count = _part_storey_count(part)
         ground_area += ground.total_floor_area_m2 or 0.0
         ground_perim += ground.heat_loss_perimeter_m or 0.0
         top_area += top.total_floor_area_m2 or 0.0
-        gross_wall += (ground.heat_loss_perimeter_m or 0.0) * part_height * part_storeys
-        party_wall += (ground.party_wall_length_m or 0.0) * part_height * part_storeys
-        total_storey_count += part_storeys
+        gross_wall += (ground.heat_loss_perimeter_m or 0.0) * part_height * part_floor_count
+        party_wall += (ground.party_wall_length_m or 0.0) * part_height * part_floor_count
         for fd in part.sap_floor_dimensions:
             fa = fd.total_floor_area_m2 or 0.0
             fh = fd.room_height_m or _DEFAULT_STOREY_HEIGHT_M
@@ -127,12 +129,16 @@ def dimensions_from_cert(epc: EpcPropertyData) -> Dimensions:
         # Figure 4) — neither path appears in current corpus, but
         # downstream calcs will silently use 2.45 m if we hit one.
         rir = part.sap_room_in_roof
+        rir_adds_storey = 0
         if rir is not None and rir.floor_area > 0:
             sum_per_storey_area_m2 += rir.floor_area
             sum_per_storey_volume_m3 += (
                 rir.floor_area * _RR_SIMPLIFIED_STOREY_HEIGHT_M
             )
-            total_storey_count += 1
+            rir_adds_storey = 1
+        part_storey_counts.append(part_floor_count + rir_adds_storey)
+
+    total_storey_count = max(part_storey_counts) if part_storey_counts else 0
 
     has_storeys = sum_per_storey_area_m2 > 0
     avg_height = (
diff --git a/packages/domain/src/domain/sap/worksheet/tests/test_dimensions.py b/packages/domain/src/domain/sap/worksheet/tests/test_dimensions.py
index 7b7e853b..e0cb085b 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/test_dimensions.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/test_dimensions.py
@@ -136,7 +136,89 @@ def test_main_plus_extension_sums_areas_perimeters_and_walls() -> None:
     assert result.gross_wall_area_m2 == pytest.approx(138.4, abs=0.05)
     # main party: 5 × 2.5 × 1 = 12.5; extension party: 0 × 2.4 × 1 = 0
     assert result.party_wall_area_m2 == pytest.approx(12.5)
-    assert result.storey_count == 2  # one storey per part, two parts
+    # SAP §2 (9) "ns": dwelling height (max across parts), NOT Σ across
+    # parts. Both parts here are single-storey, so the dwelling is one
+    # storey tall regardless of how many extensions stick out sideways.
+    assert result.storey_count == 1
+
+
+def test_dwelling_storey_count_is_max_across_parts_not_sum() -> None:
+    # Arrange — SAP §2 (9) requires the **dwelling height** for the (10)
+    # additional-infiltration adjustment, which is the tallest part, not
+    # the sum of per-part floor counts. Surfaced by Elmhurst 000474: a
+    # 2-storey main + 1-storey side extension lodges as ns=2 in the
+    # worksheet, but our pre-fix code returned 2 + 1 = 3 and over-stated
+    # (10) by 0.1 ach.
+    main = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=50.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=30.0, floor=0,
+            ),
+            make_floor_dimension(
+                total_floor_area_m2=50.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=30.0, floor=1,
+            ),
+        ],
+    )
+    extension = make_building_part(
+        identifier="Extension 1",
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=15.0, room_height_m=2.4,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=16.0, floor=0,
+            ),
+        ],
+    )
+    epc = make_minimal_sap10_epc(total_floor_area_m2=115.0, sap_building_parts=[main, extension])
+
+    # Act
+    result = dimensions_from_cert(epc)
+
+    # Assert — dwelling is 2 storeys tall, not 3.
+    assert result.storey_count == 2
+
+
+def test_room_in_roof_on_main_adds_one_to_dwelling_storey_count_only_once() -> None:
+    # Arrange — Main 2-storey with RR + same-height 2-storey extension
+    # without RR. RR adds one storey to MAIN (giving 3), extension stays
+    # at 2 storeys. Dwelling height = max(3, 2) = 3. Pre-fix code summed
+    # main-with-rr (3) + extension (2) = 5.
+    main = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=50.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=30.0, floor=0,
+            ),
+            make_floor_dimension(
+                total_floor_area_m2=50.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=30.0, floor=1,
+            ),
+        ],
+        sap_room_in_roof=SapRoomInRoof(floor_area=20.0, construction_age_band="B"),
+    )
+    extension = make_building_part(
+        identifier="Extension 1",
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=15.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=16.0, floor=0,
+            ),
+            make_floor_dimension(
+                total_floor_area_m2=15.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=16.0, floor=1,
+            ),
+        ],
+    )
+    epc = make_minimal_sap10_epc(total_floor_area_m2=140.0, sap_building_parts=[main, extension])
+
+    # Act
+    result = dimensions_from_cert(epc)
+
+    # Assert — Main with RR is 3 storeys, extension is 2 — dwelling is 3.
+    assert result.storey_count == 3
 
 
 def test_empty_sap_building_parts_uses_top_level_tfa_with_default_height() -> None:
@@ -349,16 +431,20 @@ def test_all_rir_shapes_apply_section_1_2_45m_convention_uniformly(
     result_with_rr = dimensions_from_cert(epc)
     result_without_rr = dimensions_from_cert(_strip_room_in_roof(epc))
 
-    # Assert — the RR contribution is exactly the spec convention. One
-    # storey added per part that carries a sap_room_in_roof block (a
-    # detached + extension can both have an attic conversion).
+    # Assert — TFA and volume DO sum across every RR-bearing part (genuine
+    # sums per RdSAP §3.9.1). Storey count, by contrast, is the dwelling
+    # height (max across parts) per SAP §2 (9), so RR contributes at most
+    # one extra storey to the dwelling — the storey it adds to the part
+    # that ends up tallest. All three fixtures here have single-storey
+    # parts, so any part gaining RR becomes the new tallest stack and
+    # storey_delta is exactly 1.
     tfa_delta = result_with_rr.total_floor_area_m2 - result_without_rr.total_floor_area_m2
     volume_delta = result_with_rr.volume_m3 - result_without_rr.volume_m3
     storey_delta = result_with_rr.storey_count - result_without_rr.storey_count
 
     assert tfa_delta == pytest.approx(rir_floor_area_total)
     assert volume_delta == pytest.approx(rir_floor_area_total * 2.45)
-    assert storey_delta == len(parts_with_rr)
+    assert storey_delta == 1
 
 
 from types import ModuleType  # noqa: E402  (kept near the Elmhurst tests)
diff --git a/packages/domain/src/domain/sap/worksheet/tests/test_ventilation.py b/packages/domain/src/domain/sap/worksheet/tests/test_ventilation.py
index 3db3778e..7afc1d6f 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/test_ventilation.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/test_ventilation.py
@@ -472,20 +472,24 @@ def test_section_2_matches_elmhurst_worksheet(fixture: ModuleType) -> None:
     """Real Elmhurst SAP10.2 worksheets — asserts every populated §2 line
     ref against the worksheet output for each registered fixture.
 
-    `storey_count` and `sheltered_sides` come from the fixture (Elmhurst
-    quirks: ns=3 here is dwelling height not Σ parts, sheltered_sides
-    varies per cert). The HAS_SUSPENDED_TIMBER_FLOOR flag also varies —
-    some Elmhurst assessors lodge "Suspended Timber" floor U-value while
-    ticking (12) = 0.0 (treat as effectively sealed).
+    `sheltered_sides` and HAS_SUSPENDED_TIMBER_FLOOR vary per cert and are
+    carried on the fixture. `storey_count` is now derived from
+    `dims.storey_count` (post-max-semantic fix); the LINE_9_STOREYS field
+    on each fixture cross-checks that the derivation matches the
+    worksheet's (9) value.
     """
     # Arrange
     from domain.sap.worksheet.dimensions import dimensions_from_cert
     dims = dimensions_from_cert(fixture.build_epc())
+    assert dims.storey_count == fixture.LINE_9_STOREYS, (
+        f"dims.storey_count={dims.storey_count} should equal worksheet (9) "
+        f"= {fixture.LINE_9_STOREYS}"
+    )
 
     # Act
     result = ventilation_from_inputs(
         volume_m3=dims.volume_m3,
-        storey_count=fixture.LINE_9_STOREYS,
+        storey_count=dims.storey_count,
         is_timber_or_steel_frame=False,
         intermittent_fans=fixture.INTERMITTENT_FANS,
         has_suspended_timber_floor=fixture.HAS_SUSPENDED_TIMBER_FLOOR,