Hip-to-gable loft + ASHP & solar, Wimbledon

Brief

A 1935 semi-detached in Wimbledon Park — end of a quiet cul-de-sac, not in a conservation area and outside Wimbledon's Article 4 zone — offered ideal hip-to-gable geometry. The clients wanted a master suite at roof level plus a serious EPC uplift: the existing E-rated house was costing £3,200/year in gas. Brief: hip-to-gable loft with en-suite, ASHP replacing gas boiler, and south-facing PV solar.

Challenge

Hip-to-gable conversions on 1935 semis require the party wall between the two halves of the pair to be extended vertically to form the new gable — a structural operation that triggers a Party Wall Award on both the shared wall and the rear garden boundary if a neighbour's outbuilding sits within 3m. The adjoining owner had a brick outbuilding 1.8m from the shared boundary. Two separate Party Wall notices were required; the neighbour instructed their own Surveyor (Agreed Surveyor refused), adding 12 weeks to pre-construction. ASHP location was constrained: Merton's permitted development rules for heat pumps exclude the front of the property and the side elevation visible from the highway — rear garden only, with 1m setback from boundaries.

Solution

Party Wall Agreements: shared wall Award (new gable extension) and rear boundary Award (foundation vibration from scaffold) both sealed at week 9 of pre-construction. Hip-to-gable executed by building a new blockwork gable from first-floor plate height; the original hip rafters removed; new ridge extended; full-width rear dormer 5.8m wide × 2.4m high in smooth painted render (Merton DM Policy D2 — no clay tile requirement outside CA). Master suite 24m², walk-in wardrobe, en-suite with UFH. Velux rooflight to new gable face (north elevation — borrowed light). ASHP (Vaillant aroTHERM Plus 7kW, MCS accredited) positioned in rear garden 1.4m from boundary; 8 × 400W PV panels (3.2kWp) to south-facing rear roof slope via structural bracket into new dormer cheek rafters. BUS grant £7,500 applied; SEG export tariff registered with Octopus.

Outcome

Floor area added: 24m² master suite + 6m² walk-in wardrobe. EPC improved from E (47) to B (84) — the combined effect of ASHP, PV and enhanced roof insulation (PIR 150mm between/below rafters, U-value 0.13 W/m²K). Modelled gas bill eliminated; electricity bill net of SEG export estimated £640/year vs former £3,200/year gas. Party Wall process extended pre-construction to 14 weeks — Builderr absorbed this into programme without delaying the site start. Build completed 14 weeks; total project 28 weeks from instruction.