A garage conversion in 2026 typically costs £8,000–£20,000 for a straightforward integral or attached single garage turned into a home office, playroom or extra bedroom — the most common job, because the walls, roof and foundation already exist and the work is mostly insulation, a new floor, electrics and plastering. A detached single garage runs £20,000–£31,000, more because it usually needs its own water, drainage and electricity supply run out to it. A double garage, integral or detached, runs £20,000–£45,000, and adding a bathroom or small kitchen on top of any of these adds a further £5,500–£35,000 depending on scope.
But those figures only answer what the building work costs — not whether it was done properly, which is the question I actually get asked. A garage is the one part of most houses built to none of the standards a habitable room needs: no proper damp-proof course tied into the house, no insulation to speak of, single-skin brick in plenty of older examples, and a floor slab that was never meant to have carpet laid over it. Converting one is a genuine change of use under the building regulations, and I see a steady stream of conversions — usually done years before the current owner bought — where that step was skipped entirely.
I'm a building surveyor and energy assessor, not a conversion contractor, so what follows is 2026 pricing cross-checked against current UK cost guides, what actually drives the number, the building regulations point that decides whether the finished room is an asset or a liability, and what to do if you're buying a house with a conversion that was never signed off.
The short answer
Figures below were checked on 12 August 2026 against 2026 cost guides from MyJobQuote, Checkatrade and other UK trade pricing sources, cross-referenced against each other. As with the rest of this series, London and the South East, Dom’s patch included, commonly run 15–25% above the national figures quoted here; northern regions tend to run 10–20% below.
| Type | Typical cost (2026) | What's included |
|---|---|---|
| Integral / attached single garage | £8,000–£20,000 | Wall, floor and roof insulation, new floor with damp-proof membrane, electrics, plastering, infilling or replacing the garage door opening |
| Detached single garage | £20,000–£31,000 | As above, plus a water, drainage and power supply run out from the house, and often a full re-roof |
| Double garage (integral or detached) | £20,000–£45,000 | Roughly double the floor area of a single, sometimes with a structural opening knocked through into the house |
| Add a bathroom or kitchenette | +£5,500–£35,000 | Plumbing and drainage connection, extract ventilation, fittings; the top of the range is a full kitchen for an annexe-style conversion |
Per square metre, that works out at roughly £625–£1,700/m² depending on garage type and finish level, rising to £2,400–£3,000/m² for a high-spec conversion with underfloor heating or a wet room. A typical project runs 2–4 weeks for a straightforward single garage, extending to 6–8 weeks where there's structural work, a bathroom, or utility connections to bring in from the house.
What drives the price
Garage type. Integral and attached garages are cheapest because they already share a wall, a foundation and usually a roofline with the house; detached garages cost more because every service — water, waste, power — has to be run out to them, sometimes involving groundworks across a garden.
The front opening. Infilling the garage door opening with a proper foundation, cavity wall and matching window or door typically runs £3,000–£6,000, and is not a job to build off the edge of the existing garage floor slab — it needs its own footing.
Insulation and damp-proofing. Bringing the walls, floor and roof up to a habitable standard typically costs £1,500–£5,000, and is the single biggest determinant of whether the finished room is actually pleasant to sit in, covered in more detail below.
Drawings and building control. Architectural drawings run £500–£2,500, a structural engineer for the opening or any load-bearing changes £500–£1,200, and the building control application itself typically £300–£1,000 depending on the route and your local authority's fee scale.
Bathroom or kitchen additions. A bathroom typically adds £5,500–£9,000; a full kitchen, usually only relevant for an annexe-style conversion, £15,000–£35,000.
Region. London and the South East commonly run 15–25% above the national figures quoted here, largely down to labour rates and site access.
Building regulations — the bit that decides whether it's a room or a liability
Planning permission and building regulations are two entirely separate systems, and it's the second one that actually matters here. Most garage conversions fall under permitted development for planning purposes, because they don't usually change the footprint, height or external appearance of the house much. But converting a garage from non-habitable to habitable space is always a change of use under the building regulations, whatever the planning position, and that's a mandatory application — either a building notice or full plans submission to your local authority or an approved inspector — covering structure, fire safety, ventilation, electrics, and thermal performance to the walls, floor and roof.
The thermal and damp side is where I find most of the problems. A garage floor slab was built to hold a car, not to have carpet or laminate laid on it, and most have no damp-proof membrane at all; bringing one up to standard typically means a new DPM or a tanking slurry to the floor and lower walls, lapped into the house's existing damp-proof course. That matters because a garage floor is very often lower than the house's DPC level, or simply doesn't have a DPC in the same place, which is exactly the kind of detail that gets skipped when a conversion is done quickly and cheaply. Walls are the same story: most garages are single-skin brick or blockwork with no cavity, so proper insulation means either an internal stud-and-insulation lining or external wall insulation, not just a coat of plasterboard over bare block, which is what I still find behind a surprising number of "finished" conversions.
What I actually see on survey, on the ones done without building control involvement: damp readings at skirting level within a year or two of completion, insulation that stops short at the corners and around the door opening, and no trickle vents or extract fan in a room that was never designed to breathe — condensation on the inside of the old garage door reveal is a familiar giveaway. It's the same misdiagnosis pattern I cover in damp proofing cost: what looks like rising damp in a converted garage is very often just a missing DPM and nowhere for moisture to go.
If you're buying a house with a garage conversion and there's no building control completion certificate, don't take the seller's word that it was done properly — get it checked specifically for damp and insulation as part of your survey. From there, two routes exist: regularisation, a retrospective application where the local authority inspects the work (and can require parts of it opened up) and issues a certificate if it complies, typically £400–£1,200; or indemnity insurance, a one-off policy typically £50–£300 that protects against enforcement action but doesn't confirm the work is actually sound. Most mortgage lenders will accept either, but they solve different problems — only one of them tells you the room is safe.
What I'd watch for in a quote
- Building regulations route confirmed in writing — building notice or full plans — not left as "we'll sort that", with the fee itemised separately from the build cost.
- Floor treatment specified as a damp-proof membrane or tanking system, not just "we'll damp-proof it", and how it's lapped into the house's existing damp-proof course.
- Insulation spec stated for walls, floor and roof, not described as "fully insulated" with no detail on method or thickness.
- Ventilation scoped explicitly — trickle vents or an extract fan — for a room that previously had none and won't naturally breathe like the rest of the house.
- The front opening priced with its own foundation, not built off the edge of the existing garage slab.
Questions I get asked
Does a garage conversion need planning permission? Usually not, provided the garage stays within its existing footprint and roofline and the conversion doesn't materially change the external appearance — most fall under permitted development. That's not universal: an Article 4 direction, a conservation area, a listed building, or a garage forming part of a shared wall on a semi or terrace can all remove or complicate the permitted development route, so it's worth a quick check with the local planning authority first, particularly if you're also enlarging the opening.
Does a garage conversion need building regulations approval? Yes, always, regardless of the planning position. It's a change of use from non-habitable to habitable space, which triggers building regulations covering structure, insulation, damp-proofing, ventilation, fire safety and electrics. Planning permission and building regulations are separate systems — a conversion can be entirely lawful under permitted development and still be non-compliant, and often unsafe, if building control was never involved.
Does a garage conversion add value to a house? Estimates commonly put the uplift at 10–20% of the property's value for a well-executed conversion, and it's usually the cheapest way to add habitable floor area since the walls, roof and foundation already exist. That assumes proper sign-off, insulation and damp-proofing throughout — a conversion missing any of it is routinely marked down by a surveyor rather than added to the valuation.
I'm buying a house with a garage conversion and there's no completion certificate — what do I do? Get it looked at as part of your survey, specifically for damp at the floor and abutting walls and any sign the structural opening wasn't properly supported. Then it's regularisation — a retrospective council inspection, typically £400–£1,200, that can require work to be opened up — or indemnity insurance, typically £50–£300, which insures the risk but doesn't certify the work. Most lenders accept either, but only one confirms the room is actually sound.
If you're weighing up a garage conversion, or buying a house with one already done and no paperwork to show for it, the question worth answering first is whether it was ever built to a habitable standard — that's exactly what my building survey work is built to catch. Call 07946 618203 or get in touch for a straight answer before you commit.
Small print. This guide is general information, not advice on a specific property, and it doesn’t replace a proper inspection and quote for your own home. Prices were researched and correct to the best of my knowledge on 12 August 2026; costs move and vary by region and property. Always get at least three written quotes.