diff --git a/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py b/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py
index 4960166e..e50969d1 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py
@@ -1093,6 +1093,51 @@ from domain.sap.worksheet.tests._elmhurst_fixtures import (  # noqa: E402
     ALL_FIXTURES as _ELMHURST_FIXTURES,
     fixture_id as _elmhurst_fixture_id,
 )
+from domain.sap.worksheet.tests import (  # noqa: E402
+    _elmhurst_worksheet_000474 as _w000474,
+    _elmhurst_worksheet_000490 as _w000490,
+)
+
+_NON_RR_FIXTURES: tuple[ModuleType, ...] = (_w000474, _w000490)
+
+
+@pytest.mark.parametrize("fixture", _NON_RR_FIXTURES, ids=_elmhurst_fixture_id)
+def test_section_3_non_rr_line_31_and_36_match_elmhurst_worksheet(
+    fixture: ModuleType,
+) -> None:
+    """Non-RR fixtures: (31) total external element area and (36) thermal
+    bridging match the Elmhurst worksheet exactly.
+
+    LINE_31 = Σ_parts (gross_wall + roof + floor). Window and door areas
+    cancel when expanding the net-wall term: gross_wall − w − d + w + d =
+    gross_wall. So LINE_31 is independent of window/door apportionment,
+    meaning we can assert it with zero window area and still hit the exact
+    worksheet value. LINE_36 = y × LINE_31 follows for free.
+
+    The per-storey-perimeter fix (commit e6c768c3) enabled this: before
+    that fix, gross_wall used ground_perim × avg_height × count, which
+    over-counted whenever upper storeys have a smaller perimeter than the
+    ground floor (e.g. 000474 Main: 7.07 m ground vs 5.27 m first storey).
+    """
+    # Arrange — window/door values are irrelevant for LINE_31; pass zeros
+    # to make the independence explicit.
+    epc = fixture.build_epc()
+
+    # Act
+    result = heat_transmission_from_cert(
+        epc,
+        window_total_area_m2=0.0,
+        window_avg_u_value=None,
+        door_count=0,
+    )
+
+    # Assert
+    assert result.total_external_element_area_m2 == pytest.approx(
+        fixture.LINE_31_TOTAL_EXTERNAL_AREA_M2, abs=0.01
+    )
+    assert result.thermal_bridging_w_per_k == pytest.approx(
+        fixture.LINE_36_THERMAL_BRIDGING_W_PER_K, abs=0.01
+    )
 
 
 @pytest.mark.parametrize("fixture", _ELMHURST_FIXTURES, ids=_elmhurst_fixture_id)
@@ -1139,8 +1184,8 @@ def test_section_3_partial_match_against_elmhurst_worksheet(fixture: ModuleType)
     assert result.total_w_per_k == pytest.approx(
         result.fabric_heat_loss_w_per_k + result.thermal_bridging_w_per_k, rel=1e-9
     )
-    # External-area: RR fixtures still under-count vs worksheet because RR
-    # sub-areas are not modelled (gap #1). Non-RR fixtures should match
-    # the worksheet's (31) once the per-storey-perimeter fix lands; for
-    # now keep the looser non-zero check until RR closes.
+    # External-area: RR fixtures under-count vs worksheet because RR
+    # sub-areas (gable/slope/stud-wall/flat-ceiling) are not modelled
+    # (gap #1). Non-RR fixtures get exact (31) + (36) asserted by the
+    # dedicated `test_section_3_non_rr_line_31_and_36_match_elmhurst_worksheet`.
     assert result.total_external_element_area_m2 > 0