Khalim Conn-Kowlessar ea35bed24c S0380.236: extension party-wall type read independently of "As Main Wall" ...

RdSAP 10 §3.3: "As Main Wall: Yes" makes an extension inherit the main
dwelling's external wall CONSTRUCTION only — the party wall type is
lodged separately per building part in the Summary §7 block and may
differ. `_extract_extensions` was copying `main_walls.party_wall_type`
into the inherited WallDetails, so every extension reused the main's
party wall U.

On the double_glazing fixture (Summary_001431) the Main lodges party
"CU Cavity masonry unfilled" (SAP10 wall_construction 4 → u_party_wall
0.5) but the 1st Extension lodges "U Unable to determine" (→ 0 → RdSAP
default 0.25). Pre-fix both building parts used 0.5, inflating worksheet
(32) party-wall heat loss by 6.56 W/K (Ext1 26.25 m² × 0.25). After the
fix worksheet (32) is exact: ours 32.573 vs worksheet 32.5725.

Now reads the extension's own "Party Wall Type" from its §7 chunk,
falling back to the main's only when the extension lodges none. Adds a
fixture + test asserting Main=4 / Ext=0 with distinct u_party_wall.
Suite 2413 pass; no cohort regression.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

2026-06-05 09:19:43 +00:00

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