diff --git a/backend/documents_parser/elmhurst_extractor.py b/backend/documents_parser/elmhurst_extractor.py
index 1f61fedf..e8a90d91 100644
--- a/backend/documents_parser/elmhurst_extractor.py
+++ b/backend/documents_parser/elmhurst_extractor.py
@@ -563,6 +563,7 @@ class ElmhurstSiteNotesExtractor:
                     u_value_known=main_walls.u_value_known,
                     party_wall_type=main_walls.party_wall_type,
                     thickness_mm=main_walls.thickness_mm,
+                    insulation_thickness_mm=main_walls.insulation_thickness_mm,
                     alternative_walls=self._alternative_walls_from_lines(wall_lines),
                 )
             else:
diff --git a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
index e2c32953..635b5308 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -59,6 +59,7 @@ _SUMMARY_001479_PDF = _FIXTURES / "Summary_001479.pdf"
 _SUMMARY_000897_PDF = _FIXTURES / "Summary_000897.pdf"
 _SUMMARY_000784_PDF = _FIXTURES / "Summary_000784.pdf"
 _SUMMARY_000899_PDF = _FIXTURES / "Summary_000899.pdf"
+_SUMMARY_000903_PDF = _FIXTURES / "Summary_000903.pdf"
 
 # GOV.UK EPB API JSON for cert 001479 — the API-path counterpart of the
 # Summary_001479.pdf fixture. Together they drive the API ≡ Summary
@@ -620,6 +621,35 @@ def test_summary_0380_cylinder_block_surfaces_full_15_1_lodging() -> None:
     assert epc.sap_heating.cylinder_thermostat == "Y"
 
 
+def test_summary_0350_ext1_inherits_main_wall_insulation_thickness() -> None:
+    # Arrange — cert 0350-2968-2650-2796-5255 is a multi-bp dwelling
+    # (Main + 1st Extension). Its Summary §7 Walls block lodges
+    # "1st Extension / As Main Wall / Yes" — the extension's walls
+    # inherit Main's lodgings (CA Cavity, FE Filled Cavity + External,
+    # 100 mm). The `_extract_extensions` "As Main Wall" inheritance
+    # at `elmhurst_extractor.py:559-567` builds a new WallDetails by
+    # copying Main's fields, but the field set it copies was frozen
+    # before Slice S0380.4 added `insulation_thickness_mm` — so the
+    # extension's `WallDetails.insulation_thickness_mm` falls through
+    # to its dataclass default (None), and the mapper surfaces
+    # `wall_insulation_thickness=None` on bp[1]. The cascade then
+    # routes Ext1's composite walls off the lodged-thickness path,
+    # over-stating Ext1 `external_walls_w_per_k` against worksheet
+    # line ref (29a) "External walls Ext1  5.21  0.25  1.3025".
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000903_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert — Ext1 inherits Main's 100 mm thickness and the EPC
+    # surfaces "100mm" on bp[1] (matching bp[0]).
+    assert len(epc.sap_building_parts) == 2
+    main_bp, ext1_bp = epc.sap_building_parts
+    assert main_bp.wall_insulation_thickness == "100mm"
+    assert ext1_bp.wall_insulation_thickness == "100mm"
+
+
 def test_summary_0380_full_chain_sap_within_spec_floor_of_worksheet() -> None:
     # Arrange — cert 0380-2471-3250-2596-8761 (Summary_000899.pdf /
     # dr87-0001-000899.pdf) is the first heat-pump cert under per-cert