diff --git a/domain/sap10_calculator/worksheet/heat_transmission.py b/domain/sap10_calculator/worksheet/heat_transmission.py
index d21b2eb1..dff61a08 100644
--- a/domain/sap10_calculator/worksheet/heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/heat_transmission.py
@@ -341,6 +341,13 @@ def _joined_descriptions(elements: list[Any]) -> Optional[str]:
 
 def _part_geometry(part: SapBuildingPart) -> dict[str, float]:
     if not part.sap_floor_dimensions:
+        # A part with no floor dimensions has no derivable RR shell or
+        # cantilever geometry, but the early return must still expose the
+        # SAME keys as the full return below: the §3.9 RR block reads
+        # geom["rr_common_wall_area_m2"] / ["rr_gable_area_m2"] /
+        # ["cantilever_floor_area_m2"] for every part, so omitting them
+        # here raised KeyError on multi-part certs whose first bp lodges
+        # no sap_floor_dimensions (5 certs in a 2026 API sample).
         return {
             "ground_floor_area_m2": 0.0,
             "top_floor_area_m2": 0.0,
@@ -348,6 +355,9 @@ def _part_geometry(part: SapBuildingPart) -> dict[str, float]:
             "party_wall_area_m2": 0.0,
             "rr_floor_area_m2": 0.0,
             "rr_simplified_a_rr_m2": 0.0,
+            "rr_common_wall_area_m2": 0.0,
+            "rr_gable_area_m2": 0.0,
+            "cantilever_floor_area_m2": 0.0,
         }
     fds = list(part.sap_floor_dimensions)
     ground = next((fd for fd in fds if fd.floor == 0), fds[0])
diff --git a/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py b/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
index 76edac33..600bf7d9 100644
--- a/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
+++ b/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
@@ -36,11 +36,36 @@ from domain.sap10_calculator.worksheet.heat_transmission import (
     heat_transmission_from_cert,
 )
 from domain.sap10_calculator.worksheet.heat_transmission import (
+    _part_geometry,  # pyright: ignore[reportPrivateUsage]
     _round_half_up,  # pyright: ignore[reportPrivateUsage]
     _window_bp_index,  # pyright: ignore[reportPrivateUsage]
 )
 
 
+def test_part_geometry_floorless_part_honours_full_key_contract() -> None:
+    # Arrange — a building part lodged with NO sap_floor_dimensions (e.g.
+    # a party-wall-only or RR-only extension; observed on 5 certs in a
+    # 2026 API sample). `_part_geometry`'s early return must expose the
+    # same dict keys as its full return: the §3.9 RR contribution block
+    # reads geom["rr_common_wall_area_m2"] / ["rr_gable_area_m2"] for
+    # EVERY part, so a missing key raises KeyError and blocks the cert.
+    floorless = make_building_part(floor_dimensions=[])
+    with_floors = make_building_part(
+        floor_dimensions=[make_floor_dimension(total_floor_area_m2=50.0)]
+    )
+
+    # Act
+    early = _part_geometry(floorless)
+    full = _part_geometry(with_floors)
+
+    # Assert — identical key contract; the RR/cantilever geometry is 0.0
+    # for a floorless part (no floor area ⇒ no RR shell or cantilever).
+    assert set(early.keys()) == set(full.keys())
+    assert early["rr_common_wall_area_m2"] == 0.0
+    assert early["rr_gable_area_m2"] == 0.0
+    assert early["cantilever_floor_area_m2"] == 0.0
+
+
 def test_roof_insulated_assumed_with_ni_thickness_uses_50mm_per_section_5_11_4() -> None:
     # Arrange — 346 corpus certs lodge roof_insulation_thickness="NI"
     # with descriptions like "Pitched, insulated (assumed)". The