What is included in a garage conversion cost?
A turnkey London garage conversion includes saw-cutting and replacement of the existing concrete slab with an insulated raft floor (Part L requires minimum 0.18 W/m²K, achieved with 150mm PIR over a polythene damp-proof membrane), full insulation of walls (50mm-70mm PIR on internal blockwork or full external wall insulation), insulation of the ceiling/roof to 0.16 W/m²K, brick infill of the garage door opening with matching stock brick or rendered blockwork, new window or French door to the front, internal stud partitions if creating multiple rooms, electrical first and second fix with consumer unit upgrade if required, heating extension from existing boiler or installation of electric radiators or underfloor heating, plasterwork, flooring, decoration. Items frequently excluded: structural engineer for any roof or door opening changes (£400-£800), Building Control fees (£300-£600), new sewer connection if creating a bathroom (£1,500-£3,500), kitchen install if creating a self-contained unit (£3,500-£15,000).
Garage conversion cost by type
Three garage configurations dominate London. Integrated single garage (attached to house, sharing walls): £18,000-£28,000 for a 14-16m² standard space. Attached double garage or integrated double: £28,000-£40,000 for 28-32m². Detached garage (standalone block in garden or driveway): £25,000-£40,000 due to extending heating, electrics and water 8-15m to the new structure, plus weatherproofing the previously external walls. Conversion to self-contained annexe with kitchen and bathroom adds £8,000-£15,000 to base price; conversion to office, gym or playroom only is cheapest. Loft over existing garage adds further £15,000-£25,000 where head height allows. Most popular London use cases: home office (45% of conversions), extra bedroom (25%), playroom or family room (20%), self-contained annexe (10%).
Planning and permitted development
Most integrated and attached garage conversions in London are permitted development under Schedule 2 Part 1 Class A or Class C, provided the conversion does not extend or enlarge the building footprint and external alterations (such as brick infill of the garage door) use matching materials. PD does not apply in conservation areas, on listed buildings, on flats, or where Article 4 directions have been imposed (covering parts of Hackney, Camden, Islington and other inner boroughs). A Lawful Development Certificate (£103 fee, 6-8 weeks) is strongly recommended to confirm PD status before works begin. Detached garage conversion sometimes requires planning where the structure is changing use class or where the original garage was an outbuilding rather than a habitable building. Always confirm with the local planning authority before commissioning works.
Realistic London timelines
On-site duration for a typical London garage conversion is 4-8 weeks. A like-for-like office conversion with no plumbing completes in 4-5 weeks; an annexe conversion with kitchen and bathroom 7-9 weeks. Lead times before site start are usually short: 2-3 weeks for LDC if required, 1-2 weeks for design and survey, 1 week for material orders. Plan total project duration of 10-14 weeks from instruction to handover. Detached garages with extending services or roof remediation take 8-12 weeks on site. The most common delay cause is unexpected damp under the existing slab — investigation and remediation can add 1-2 weeks and £1,500-£4,000 in additional works.
