1928 detached garage → self-contained 1-bed annexe with kitchenette + shower room

Brief

1928 detached double-garage outbuilding (38m² footprint, 4.2m ridge height, pitched roof, brick + render walls) on a 0.4-acre TW1 St Margarets plot adjacent to a 1928 Arts-and-Crafts detached main house. Owner-occupier downsizing parents-in-law from a 4-bed detached in Surrey + want self-contained accommodation on the same plot to enable shared childcare. Brief: full conversion of the garage to a self-contained 1-bed annexe — open-plan living/dining/kitchenette zone + sleeping alcove + shower room — with C3 ancillary use (not separate dwelling) to avoid CIL liability + business-rate exposure. Heritage-aware spec to match the Arts-and-Crafts main house aesthetic.

Challenge

Six issues. (1) Use class — self-contained annexe must read as C3 ancillary use (occupied by family member dependent on main household) rather than C3 separate dwelling (would trigger CIL ~£18k + business-rates exposure + Council Tax Band A standalone). Planning condition wording critical. (2) St Margarets Conservation Area + Thames Riverside Conservation Area buffer — front-elevation alterations restricted; main visible elevation faces the side road. (3) Existing 1928 garage walls — single-skin 215mm solid brick (no cavity) + render externally — required internal liner build-up reducing internal floor area; alternatively external wall insulation EWI would change external appearance triggering CO refusal. (4) Roof — existing 1928 pitched timber roof 4.2m ridge height with clay-tile finish on softwood rafters 75×125mm at 400mm centres; required full rafter insulation upgrade + structural sister-rafter check. (5) Flood Zone — TW1 St Margarets sits in Flood Zone 2 (Thames tidal); new habitable floor levels must be raised + flood-resilient construction. Environment Agency Flood Risk Assessment required for any new dwelling-equivalent; ancillary annexe consent route differs. (6) Drainage — existing garage had no foul drainage; new soil branch + foul connection to mains required + Thames Water Build-Over Agreement where new branch crosses the public sewer easement (200mm clay sewer running 1.8m below the side path).

Solution

Pre-application engagement Richmond Planning Service week -16 (£300 fee). CO confirmed annexe ancillary use acceptable subject to (a) physical/functional link to main house (covered walkway built between main house side door + annexe entrance), (b) no separate utility metering (annexe metered through main house sub-meter only), (c) Section 106 unilateral undertaking restricting use to ancillary occupation by family member of main house occupier. Full planning application + Heritage Statement + Flood Risk Assessment + Thames Water Build-Over Agreement application submitted week -10. Approval week -2 (8-week determination + zero amendments). Build 11 weeks. Week 1 strip-out + roof tile strip-and-set-aside (clay tiles inspected + 78% retained for re-use) + boundary protection. Weeks 1–2 floor build-up — existing 1928 garage slab broken out + 600mm dig + 200mm well-compacted Type 1 + 1200g DPM + 150mm Celotex GA4000 PIR + 75mm reinforced screed + Wunda 16mm UFH; new finished floor level raised 200mm above flood zone reference level (Environment Agency Thames tidal 1-in-100-year + climate-change uplift). Weeks 2–3 wall liner build-up — internal block-and-render wall liner on existing single-skin 1928 brick walls: 50mm cavity (drained at base for flood resilience) + 120mm Celotex GA4000 PIR + service zone + 12.5mm plasterboard skim — U-value 0.18 W/m²K. Weeks 3–4 roof upgrade — sister rafters C24 47×195mm bolted to original 75×125mm rafters + 200mm Rockwool Flexi between + 50mm Celotex insulation board below + breathable membrane Tyvek Supro + reslated with 78% original 1928 clay tiles + 22% Imerys handmade clay-tile match. Two new conservation-style Velux GGL roof windows 780×980mm to north slope (Richmond CO approved — not visible from public realm). Weeks 4–6 first-fix M&E. New 100A consumer-unit + sub-meter from main house (no separate utility supply). New 110mm soil branch routed through side path void + connecting to mains via Thames Water Build-Over Agreement (£1,250 fee + insurance certificate) — 200mm bedded crushed pea-shingle + concrete encasement over the existing sewer crossing. New mains water + cold-water tank in roof void + Megaflo unvented hot-water cylinder. New ASHP Vaillant aroTHERM Plus 5kW (BUS grant £7,500 approved) with discreet Builderr-fabricated timber acoustic screen at boundary 32 dB(A) at 1m. MVHR Zehnder ComfoAir Q350 with 90% heat recovery. Weeks 6–8 internal partitions + finishes. Acoustic separation between sleeping alcove + living zone — 100mm timber stud + 2× 15mm acoustic plasterboard + Rockwool RWA45 + Genie clip-on bars Rw 50dB. Shower room 3.8m² Schlüter-KERDI tanking + KERDI-DRAIN linear drain + 600×600mm Carrara marble floor + walls + Burlington traditional thermostatic shower + bespoke single-basin oak vanity + Burlington high-level WC. Open-plan living/dining 22m² engineered oak 240mm wide-plank over UFH + Farrow & Ball 'Stiffkey Blue' feature wall + Bulmer 'Pale Hone' to remaining + Original BTC + Vaughan brass pendants + bespoke built-in bookcase wall by Naked Doors. Kitchenette 4.2m run Plain English Osea range (devolved spec — Plain English's secondary range) painted F&B 'Pigeon' + Calacatta Viola worktop + Quooker tap + Bosch integrated fridge + Neff combi-oven + induction hob 60cm + extractor. Sleeping alcove 8.5m² built-in wardrobe wall + reading nook with integrated low-level lighting + acoustically separated from living zone. Weeks 9–10 covered walkway between main house side door + annexe entrance — Builderr-fabricated oak-frame + glazed walls + clay-tile pitched roof matching main house — provides physical/functional link satisfying planning condition. Weeks 10–11 external — render touch-up + reclaimed-tile roof completion + landscaping reinstatement + commissioning + air-tightness 3.8 m³/h/m² @50Pa + Building Control final + Section 106 unilateral undertaking executed + sub-meter installation + handover.

Outcome

Planning approval week -2 (zero amendments). Section 106 unilateral undertaking executed + registered. Thames Water Build-Over Agreement issued + paid. BUS grant £7,500 received. Project completed Q3 2026, on programme. Parents-in-law moved in week 12. The 38m² self-contained annexe (22m² open-plan living/dining/kitchenette + 8.5m² sleeping alcove + 3.8m² shower room + circulation) functions as ancillary accommodation to the main house — single mains supply, single Council Tax, no CIL, no business-rates exposure, no separate utility metering. Covered walkway provides the physical/functional link required by the S106. Heritage-aware spec matches the 1928 Arts-and-Crafts main house — 78% original clay-tile retention + reclaimed-brick boundary repairs + traditional brassware + Plain English kitchenette + period detailing throughout. Energy performance: airtightness 3.8 m³/h/m² @50Pa (60% better than Part L), ASHP + MVHR + 150mm PIR floor + 120mm PIR walls + 250mm rafter insulation, predicted SAP 91 (EPC B). Resale agent (Anthony Pepe Twickenham) valued the uplift at £215,000 against £92,500 spend = 132% gross ROI — the self-contained annexe + multigenerational-ready specification differentiating from peer TW1 stock. Builderr first formally-S106-bound ancillary-annexe conversion + first Thames Water Build-Over Agreement on a garage-conversion project + first BUS grant on a detached-garage conversion + first Plain English Osea kitchenette in an annexe. Richmond upon Thames 4th portfolio case study + St Margarets 2nd.