diff --git a/domain/sap10_calculator/worksheet/heat_transmission.py b/domain/sap10_calculator/worksheet/heat_transmission.py
index e232e149..eedb95c2 100644
--- a/domain/sap10_calculator/worksheet/heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/heat_transmission.py
@@ -212,6 +212,17 @@ def _window_bp_index(window_location: Any, num_parts: int) -> int:
         for prefix, idx in (("1st", 1), ("2nd", 2), ("3rd", 3), ("4th", 4)):
             if s.startswith(prefix):
                 return idx if idx < num_parts else 0
+        # Bare "Extension" — PDF wrap artifact: cert 9380's Summary lodges
+        # "1st Extension" but pdftotext wraps "1st" onto a preceding layout
+        # line, so the extractor only captures "Extension". RdSAP10 §3 p.17
+        # requires window/door areas to deduct from the building part the
+        # opening pierces ("for each building part, software will deduct
+        # window/door areas contained in the relevant wall areas"); without
+        # this route the ext1 windows deduct from main, over-counting the
+        # main wall's contribution and under-counting the ext1's by the
+        # U-value difference × opening area.
+        if s == "extension" and num_parts >= 2:
+            return 1
     return 0
 
 
diff --git a/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py b/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py
index b26cd227..d0da32a4 100644
--- a/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/tests/test_heat_transmission.py
@@ -35,6 +35,9 @@ from domain.sap10_calculator.worksheet.heat_transmission import (
     HeatTransmission,
     heat_transmission_from_cert,
 )
+from domain.sap10_calculator.worksheet.heat_transmission import (
+    _window_bp_index,  # pyright: ignore[reportPrivateUsage]
+)
 
 
 def test_roof_insulated_assumed_with_ni_thickness_uses_50mm_per_section_5_11_4() -> None:
@@ -593,6 +596,42 @@ def test_walls_w_per_k_uses_sum_of_per_storey_perimeter_times_height_not_ground_
     assert result.walls_w_per_k == pytest.approx(24.0, abs=0.5)
 
 
+def test_window_bp_index_routes_bare_extension_to_first_extension_per_rdsap10_section_3() -> None:
+    # Arrange — RdSAP10 §3 p.17: "for each building part, software will
+    # deduct window/door areas contained in the relevant wall areas".
+    # Cert 9380's Summary PDF lodges 2 windows on its single extension,
+    # but pdftotext wraps "1st" onto a preceding layout line while
+    # "Extension" lands on a separate line — the Elmhurst extractor
+    # captures only the second token ("Extension") and the dwelling has
+    # exactly 2 building parts (Main + Ext1). Without this route the
+    # extension windows deduct from the main wall and the U-weighted
+    # net-area sum drifts by the U-difference × opening area (cert
+    # 9380: 1.59 × (0.70 − 0.53) = 0.27 W/K on (29a)).
+
+    # Act
+    bare_extension_idx = _window_bp_index("Extension", num_parts=2)
+    ordinal_idx = _window_bp_index("1st Extension", num_parts=2)
+    main_idx = _window_bp_index("Main", num_parts=2)
+
+    # Assert — bare "Extension" routes to BP[1] just like "1st Extension".
+    assert bare_extension_idx == 1
+    assert ordinal_idx == 1
+    assert main_idx == 0
+
+
+def test_window_bp_index_bare_extension_with_only_main_bp_falls_back_to_main() -> None:
+    # Arrange — when the dwelling has no extension BP at all, the
+    # "Extension" string is unresolvable; the function must fall back
+    # to BP[0] (main) rather than raise. Mirrors the existing ordinal-
+    # prefix out-of-range behaviour.
+
+    # Act
+    idx = _window_bp_index("Extension", num_parts=1)
+
+    # Assert
+    assert idx == 0
+
+
 def test_main_plus_extension_sums_per_element_contributions() -> None:
     # Arrange — Main + single-storey age L extension. Each contributes to the
     # element totals. With_extension > main_only on every populated field