Garage Conversion London 2026: Permitted Development, What the Work Involves and the Fixed Price From £21,600
By Anderson & the LCCL Construction Team | Finchley N3
An attached garage is the space most semis already have and do not use for a car. Converting it into a room — a home office, a downstairs bedroom and shower room, a bigger kitchen, a playroom — changes the outside of the house only where the up-and-over door was, and the planning rules treat it as internal work. On most houses that makes it permitted development: no planning application, a Lawful Development Certificate as proof, and a build of three to five weeks.
This guide covers when a garage conversion is and is not permitted development, what the garage structure actually needs to become a habitable room, how building control looks at it, and what the fixed price covers — from £21,600 inc. VAT (£18,000 + VAT) for a single attached garage turned into one habitable room.
Is a garage conversion permitted development?
Usually, yes. Converting the inside of an attached garage and replacing the garage door with a wall and window counts as alteration of the existing house, which is permitted development on a house outside a conservation area. There are three exceptions we check before drawing:
A planning condition on the original permission for the house requiring the garage to stay as parking — common on estates built from the 1980s onwards, and on some infill developments. The council's planning history for the address tells us; if the condition exists, the conversion needs a planning application to remove it.
Conservation areas and Article 4 streets, where changing the front elevation can need consent.
Detached garages, which are a change of use of an outbuilding rather than an alteration to the house and are judged differently — usually still fine for incidental use, not for a separate dwelling.
Flats and maisonettes have no permitted development rights, so a garage belonging to a flat needs an application. We apply for the Lawful Development Certificate on every garage conversion that qualifies, and include the council fee.
What the garage needs to become a room
A garage was built to keep a car dry, not a person warm. Building regulations treat the conversion as creating a habitable room, and four parts of the structure almost always need work:
The floor: a garage slab is usually 100 mm of concrete with no damp-proof membrane and no insulation, often below the house floor level. The conversion adds a membrane, insulation and a new floor, and brings the level up to meet the hall.
The walls: a single-skin brick wall will not meet the insulation or damp standard, so it is either dry-lined with an insulated stud wall inside or, on a cavity wall, insulated within the cavity. The wall shared with the house is checked for sound and fire.
The roof: a flat felt roof on a garage is rarely insulated. It is insulated from inside, or re-covered with an insulated warm roof if the felt is at the end of its life — which on a 1930s garage it usually is.
The front wall: the garage door opening is bricked up on a new foundation strip, or partly bricked with a window. Matching brick keeps it inside the permitted development materials rule and stops the house looking patched.
On top of that: electrics to the new room, heating extended from the house system, ventilation, and fire-door and smoke-alarm requirements where the room opens onto the hall. If the new room is a bedroom or shower room, drainage and a means of escape are added to the list.
How building control checks it
We submit full-plans drawings through LABC so the inspector has agreed the floor build-up, wall insulation, roof and the structural opening before work starts, then inspects at the foundation strip, the damp-proofing, the insulation and completion. The completion certificate is what a buyer's solicitor asks for; a garage conversion without one is the most common building-regulations gap we see on houses being sold.
What a garage conversion costs in 2026
Inc. VAT with the ex-VAT figure in brackets, economy specification, design and build inclusive, postcode dependent:
Single attached garage to one habitable room: from £21,600 inc. VAT (£18,000 + VAT), typically £21,600 – £36,000 inc. VAT (£18,000 – £30,000 + VAT) across the 12–20 m² most single garages measure. The low end is a sound slab and a cavity wall; the top end is a floor that comes up, a single-skin wall lined inside and a flat roof re-covered.
Included: survey and planning-history check, drawings, building-regulations submission through LABC, the Lawful Development Certificate, the insulated floor, wall lining or cavity insulation, roof insulation, the new front wall and window, electrics, heating extended from the house, plaster and decoration to economy specification, inspections and the completion certificate.
Priced as add-ons: a shower room or en-suite, a kitchen, drainage for either, a replacement roof, and a higher finish level — agreed on the call.
For what the market quotes: Checkatrade's 2026 guide puts a 16 m² garage conversion at £10,000 – £20,000 + VAT nationally, and the London trade guides add 15 – 25% to that. Our figure sits inside the London range and, unlike the £10,000 headline, it includes the design, the certificate, the building control file and the front wall. Run the instant estimate for a range on your postcode and garage size. The side extension guide shows the alternative when the garage is better knocked down and rebuilt as a wider extension.
Anderson's advice
“Budget for the steel before you budget for the kitchen.” Garage conversions rarely need steel, which is why people assume they cost very little — and then the survey finds the slab has to come up and the roof has to come off. Spend the first money on knowing the floor, the walls and the roof; the room is the easy part. A garage that needs all three is still a good conversion, it is just not a small one, and we would rather tell you that on the survey than on an invoice.
Why clients choose us
As LABC Building Excellence Award Winners 2024 for Best Residential Extension in London — and London Regional Winners again in 2026 for Best Conversion or Alteration to an Existing Home — we design, certify and build under one contract, with a 12-month workmanship guarantee and a 10-year LABC structural warranty. Finchley N3 based, covering London and Hertfordshire — see the first-floor extension guide for building over a garage instead, and the house extension cost guide for the extension alternatives. Already have drawings? We price them as drawn.
What our clients say
“LCCL Constructions was professional, trustworthy and worked to the highest standard. The process was smooth and I'm very happy with the result - it has transformed the quality of my home.”
— KashImaging, Trustpilot
Frequently asked questions
Do I need planning permission to convert my garage?
Usually not: converting an attached garage is internal work and permitted development on a house outside a conservation area. The exception is a planning condition on the original permission requiring the garage to stay as parking — common on newer estates — which needs an application to remove. We check the planning history before drawing.
How much does a garage conversion cost in London?
From £21,600 inc. VAT (£18,000 + VAT) for a single attached garage turned into one habitable room at economy specification, design and build inclusive, with the Lawful Development Certificate and building regulations included. Shower rooms, kitchens and a replacement roof are priced as add-ons; figures are postcode dependent.
How long does a garage conversion take?
Three to five weeks on site for a single garage once the drawings and building-regulations approval are in place, longer if the roof is replaced or a shower room is added. Design and approvals take 6–8 weeks before that.
Get a figure for your own project
To book a free 30-minute consultation call about your garage conversion, call LCCL Construction on 020 8037 7332 or get an instant estimate online — six questions, about two minutes, and you see a real range for your postcode on screen. Finchley N3 based, covering all of London and Hertfordshire.
One team, one contract, one point of contact — from first sketch to handover.
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