diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 80a650515..5a04e5165 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -4423,28 +4423,47 @@ _API_FLOOR_CONSTRUCTION_TO_STR: Dict[int, Optional[str]] = {
 # enum (1=Flat, 3=Pitched no-access, 5=Vaulted, etc.) is mapped as
 # best-effort against SAP10 nomenclature.
 #
-# Codes 6 and 7 → None. This field is read ONLY for the sloping-ceiling
-# inclined factor; the base roof U-value comes from the global
-# roofs[].description, so a non-sloping code carries no information the
-# cascade consumes here, and None correctly avoids the cos(30°) false-
-# trigger:
+# Code 6 → None. This field is read ONLY for the sloping-ceiling inclined
+# factor; the base roof U-value comes from the global roofs[].description,
+# so a non-sloping code carries no information the cascade consumes here:
 #   6 = "Thatched, with additional insulation" — its U is set by the
-#       global description; not a sloping ceiling.
-#   7 = "(same dwelling above)" / "(another dwelling above)" — an
-#       internal ceiling with no roof heat loss (the roof-side analogue
-#       of floor_construction code 0). Heat loss is governed by the
-#       roof_heat_loss / description path, not this field.
-# Empirically inert: roof W/K is identical whether 6/7 map to None or to
-# an explicit pitched string across all code-6/7 certs in the 2026
-# sample (were raising UnmappedApiCode, blocking the cert).
+#       global description; not a sloping ceiling. None correctly avoids
+#       the cos(30°) false-trigger.
+#
+# Codes 7 and 9 → "(another dwelling above)", NOT None. RdSAP 10
+# Specification (10-06-2025) §5.2.5 (p.31): "If a dwelling or part of a
+# dwelling has commercial premises above record as another dwelling
+# above" — i.e. premises-above (code 9) gets the SAME treatment as
+# dwelling-above (code 7), not the semi-exposed "partially heated space"
+# treatment used for premises *below* (see `_API_FLOOR_HEAT_LOSS_ABOVE_
+# PARTIALLY_HEATED`) — the spec's above/below split is asymmetric. Per
+# spec p.~55: "There is no heat loss through the roof of a building part
+# that has the same dwelling above or another dwelling above" — zero
+# heat loss, mirrored by the landlord-override system's independent
+# `roof_party_ceiling_guard` (`domain/epc/property_overrides/`), which
+# already resolves this exact "(another premises above)" marker to a
+# ~0-heat-loss RoofType citing the same spec line.
+#
+# `heat_transmission.py`'s per-part suppression (`part_roof_is_party =
+# "another dwelling above" in roof_type`) only fires when this field
+# CONTAINS that literal substring — it existed already for the Elmhurst
+# path (which sets it via `_strip_code(roof.roof_type)`) but the API path
+# previously mapped 7 (and, before this fix, 9) to None, so the
+# suppression never fired for ANY API-derived cert: `has_exposed_roof`
+# (the dwelling_type-label-only fallback) was the sole gate, and it has
+# no way to know about a per-part party-ceiling lodgement. Surfaced by
+# live cert uprn 10013320122 ("44 Regent House", code 9); code 7 has the
+# same latent gap (11 building parts in the RdSAP-21.0.1 corpus), fixed
+# here too rather than left half-fixed.
 _API_ROOF_CONSTRUCTION_TO_STR: Dict[int, Optional[str]] = {
     1: "Flat",
     3: "Pitched (slates/tiles), no access to loft",
     4: "Pitched (slates/tiles), access to loft",
     5: "Pitched (vaulted ceiling)",
     6: None,
-    7: None,
+    7: "(another dwelling above)",
     8: "Pitched, sloping ceiling",
+    9: "(another dwelling above)",
 }
 
 
diff --git a/datatypes/epc/domain/tests/test_from_rdsap_schema.py b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
index 7d68b4a1f..f6ce46bfb 100644
--- a/datatypes/epc/domain/tests/test_from_rdsap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
@@ -1339,18 +1339,18 @@ class TestDefaultMissingPostTown:
 
 class TestApiRoofConstructionCode:
     """`_api_roof_construction_str` maps the GOV.UK API integer
-    roof_construction code to the string the cascade reads ONLY for the
-    "sloping ceiling" cos(30°) inclined-surface factor (Slice 89). Codes
-    6 and 7 are neither sloping ceilings nor base-U drivers (the roof
-    U-value comes from the global roofs[].description), so both map to
-    None: code 6 = "Thatched" (its U is set by the description, not this
-    field) and code 7 = "(same/another dwelling above)" — an internal
-    ceiling with no roof heat loss, the roof-side analogue of
-    floor_construction code 0. Empirically inert: roof W/K is identical
-    whether 6/7 map to None or to an explicit pitched string across all
-    code-6/7 certs in the 2026 sample."""
+    roof_construction code to the string `heat_transmission.py` reads for
+    two purposes: the "sloping ceiling" cos(30°) inclined-surface factor
+    (Slice 89), and — for codes 7/9 — the "another dwelling above"
+    substring that suppresses that building part's roof entirely (zero
+    heat loss). Code 6 = "Thatched" is neither: its U-value comes from
+    the global roofs[].description, not this field, so it maps to None
+    (avoids the cos(30°) false-trigger). Codes 7 and 9 both map to
+    "(another dwelling above)" per RdSAP 10 Specification (10-06-2025)
+    §5.2.5: commercial premises above (code 9) is recorded identically to
+    another dwelling above (code 7) — both zero heat loss."""
 
-    def test_code_7_same_dwelling_above_maps_to_none(self) -> None:
+    def test_code_7_same_dwelling_above_suppresses_roof_heat_loss(self) -> None:
         # Arrange
         from datatypes.epc.domain.mapper import (
             _api_roof_construction_str,
@@ -1359,9 +1359,22 @@ class TestApiRoofConstructionCode:
         # Act
         result = _api_roof_construction_str(7)
 
-        # Assert — None: no sloping-ceiling signal (avoids the cos(30°)
-        # false-trigger); the internal ceiling has no roof heat loss.
-        assert result is None
+        # Assert — the party-ceiling marker, so heat_transmission.py's
+        # per-part "another dwelling above" suppression fires.
+        assert result == "(another dwelling above)"
+
+    def test_code_9_another_premises_above_suppresses_roof_heat_loss(self) -> None:
+        # Arrange — RdSAP 10 Spec §5.2.5: commercial premises above is
+        # recorded as "another dwelling above", same as code 7.
+        from datatypes.epc.domain.mapper import (
+            _api_roof_construction_str,
+        )  # pyright: ignore[reportPrivateUsage]
+
+        # Act
+        result = _api_roof_construction_str(9)
+
+        # Assert
+        assert result == "(another dwelling above)"
 
     def test_code_6_thatched_maps_to_none(self) -> None:
         # Arrange