What a detached garage conversion involves
A detached garage is structurally independent from the main house — it has its own walls, roof, slab and (usually) its own access. Converting it to habitable use involves the same thermal and acoustic upgrade as an integral conversion — insulate slab, walls and roof, install floor boarding, board and plaster walls — but additionally requires running new utility connections from the main house. Electricity is typically run in armoured cable buried to 450mm depth from the main consumer unit. Central heating connection is either via extending the main house system (run in a lagged insulated duct underground) or a separate electric heating system. Water supply for a WC or kitchenette is run underground from the main cold supply. All underground runs need to be accounted for in the build cost and programme.
Planning and change of use
Converting a detached garage to a non-residential use (home office, studio, gym) is almost always permitted development — provided the building doesn't exceed the maximum permitted outbuilding size (50% of garden curtilage, maximum 4m ridge height for dual-pitched roofs, 3m for any other roof). Converting to a self-contained living space (bedroom, annexe, holiday let) is a material change of use and typically requires full planning permission. The key planning question is whether the annexe would be used as a separate planning unit (which creates a new dwelling, with different rules) or as an ancillary use to the main house (a family annexe or granny flat, which is normally approvable). We advise on the most appropriate planning route at survey and submit the application on your behalf where full permission is needed. Building regulations apply to all habitable-use conversions.
Services connection from main house
The most common services run in a detached garage conversion are: electricity (armoured SWA cable from the main consumer unit — size depends on required load, typically 6mm² for a studio, 10mm² or 16mm² for a heated annexe with kitchen appliances); cold water (15mm MDPE from the mains supply, buried to 750mm to prevent freezing — we install an isolating stopcock accessible from the garden); and drainage (110mm clay or PVC drain connecting to an inspection chamber on the main house's existing drain system, gradient minimum 1:40). We coordinate all three services from a single trench wherever the garden layout allows, to minimise excavation and reinstatement. Reinstatement of paved areas, driveways or planted areas is included in our fixed-scope quote.
Thermal upgrade and insulation
Older detached garages in London are typically single-skin brick or block with a timber truss or flat roof — built to house a car, not to retain heat. Converting to habitable use requires upgrading the thermal envelope to current building regulations standards. Wall insulation: 75mm PIR boards on treated timber battens or metal channels across all four walls, achieving a U-value below 0.35 W/m²K. Roof insulation: 100mm PIR between rafters plus 50mm continuous board below to eliminate cold bridging, or 150mm insulation above ceiling board on flat roofs. Floor insulation: 75mm PIR on the existing slab under a floating ply or screed finish. Single-skin brick walls are sometimes insufficient to carry internal insulation without surface condensation risk — we thermal-model this at design stage and advise on whether external cladding or a structural upgrade to the cavity is more appropriate.
Annexe design and self-containment
A self-contained annexe in a detached garage typically contains: a sleeping area (single or double, with fitted wardrobe under the eaves if possible), a shower room (compact 900mm × 900mm walk-in shower, WC, basin), a kitchenette (base units, worktop, sink, under-counter fridge, 2-ring induction hob, microwave), and a living area. On a standard single-car garage (5m × 2.4m net internal) this is achievable but tight — the sleeping area and kitchenette share an open-plan zone. On a double-car garage (5m × 5m or wider) a generous one-bed annexe with separate bedroom is easily accommodated. The annexe entrance is kept separate from the main house to maintain privacy for both the occupant and the main household.
Heating options
The two most practical heating options for a detached garage conversion are: (1) extend the main house central heating system via underground flow-and-return pipes — cost-effective if the garage is within 15m of the house and the existing boiler has spare capacity; or (2) install an independent electric heating system — either infrared panels (no flow/return pipes, instant heat, easy to install) or an air-source heat pump (more expensive to install but lower running cost over time, eligible for government grants). On well-insulated conversions with solar exposure, a small air-source heat pump covering both heating and hot water is the most energy-efficient option and we offer this as a specification upgrade on annexe projects.
Cost drivers and variables
The largest cost variables in a detached garage conversion are: (1) distance from the main house to the garage (every 5m of service run adds roughly £1,000–1,500 in trench excavation, cable, pipe and reinstatement); (2) drainage connection — if the garage drain needs to run more than 10m to an inspection chamber, an uplift pump may be needed; (3) the floor level relative to adjacent ground (a below-grade slab needing significant floor build-up adds insulation and screed costs); (4) the extent of the thermal upgrade required (single-skin brick walls in poor condition may need recladding or replacement); and (5) the level of fit-out specified (studio versus full annexe with bathroom). Our quotations itemise each so you can make informed scope decisions.
