Garage conversion or an extension: which makes sense?
If you have a garage you no longer use for the car, converting it is almost always the cheaper way to gain a room — from around £8,000, against £24,900–£50,200 for a single-storey extension. But a conversion doesn’t make your house any bigger. It makes space you already own usable, and that distinction decides which one is right for you.
The difference that decides it
An extension adds floor area — a new footprint joined to the house. A garage conversion adds none of it; it takes a cold, often single-skin store and turns it into a warm, dry room. Both give you somewhere new to work, sleep or sit, but they are not the same project and they don’t suit the same job.
So the first question isn’t cost. It’s what you actually need. If the honest answer is a home office, a playroom, a utility or a downstairs bedroom, and you have an integral or attached garage standing empty, a conversion gets you there for a fraction of the price and far less mess. If the answer is a bigger kitchen, an open-plan living space, or simply more house, a conversion can’t deliver it — you need the extra footprint only an extension gives.
Everything below assumes an integral or attached garage: one built into the house or joined to it. A detached garage is a different job, closer to converting an outbuilding, with its own planning and building-control questions.
What each one costs
The gap is large, and it’s the main reason people convert. A garage conversion reuses walls, a roof and foundations you’ve already paid for, so you’re buying insulation, a new floor build-up, an infill wall where the door was, and the fit-out — not a whole new structure.
| Garage conversion | Single-storey extension | |
|---|---|---|
| Typical cost, built and finished | from £8,000 (single); £15,000+ (double) | £24,900 – £50,200 |
| Cost per m² | £900 – £1,800 | usually higher, and varies widely |
| Adds floor area? | No — repurposes existing space | Yes — a new footprint |
| Planning | usually permitted development if internal | permitted development within rear or side limits |
| Off-street parking | you may lose it | kept, unless you build over it |
Cost ranges: BestBuilders’ 2026 regional cost model — published national cost guides adjusted by a regional labour index, averaged across 519 UK towns. Indicative, not quotes. Planning points from the Planning Portal. For a full price breakdown see the BestBuilders garage conversion cost guide and our own house extension cost page.
One number the table hides is what you pay per useful room. Convert a single garage and you might spend £10,000–£15,000 for one finished room. Extend, and the same room arrives attached to groundworks, a steel over the old rear wall, new drainage and glazing — which is why a modest extension rarely starts under £25,000.
How much space you really gain
This is where conversions quietly disappoint people. A standard single garage is only about 2.4–2.6m wide inside, and once you’ve added insulated stud walls, a damp-proofed floor and a plastered finish, you lose 100–150mm off each surface. A garage that measured 2.5m by 5m on the tape can hand you a usable room closer to 2.2m by 4.7m — fine for an office or a snug, tight for a double bedroom with wardrobes.
An extension, by contrast, you size to the need. If the room you want simply won’t fit inside the garage footprint, that settles it: convert for what a garage can hold, extend for anything larger.
Do you need planning permission?
For most garage conversions, no. The Planning Portal’s position is that planning permission is not usually required “providing the work is internal and does not involve enlarging the building”. Converting the inside of an attached garage rarely enlarges anything, so it tends to fall under permitted development.
Two things catch people out. First, permitted development rights are often removed by a planning condition — common on newer estates and in conservation areas, sometimes specifically to keep garages as parking. If that condition sits on your house, you’ll need an application even for an internal conversion, so check before you spend anything. Second, filling in the garage door changes the front elevation, and some councils treat a significant change to the street-facing frontage as needing permission. A quick call to your local planning authority settles both.
An extension has its own route: it is permitted development only within the rear and side limits for depth, height and coverage set out in our guide to permitted development rules. Go beyond them and you are into a full application. So the planning question usually favours the conversion — but not always.
Building regulations apply either way
Skipping planning doesn’t mean skipping Building Control. A garage was never built to live in, so a conversion has to be brought up to habitable standard, and that work is exactly what building regulations check:
- The floor. Garage floors sit lower than the house and usually have no damp-proof membrane or insulation. Both have to be added, which is often why the finished floor rises.
- The infill wall. Where the door was, a new insulated wall goes in, and Building Control will want to know the ground beneath the old opening can carry it.
- Insulation, damp and ventilation to the walls and roof, plus condensation control.
- Fire safety. If the garage is integral, there are rules on separation from the rest of the house, and any door into the house may need to be a fire door.
- Electrics. New circuits are notifiable work under Part P.
An extension goes through the same regime for structure, insulation, drainage and fire. Neither project is exempt; both need Building Control sign-off. Our page on building regulations for extensions covers what gets inspected and when.
When a conversion wins — and when it doesn’t
Convert if you have a garage you don’t need for the car, you want one extra room rather than a bigger house, and the room you have in mind fits inside the existing walls. It is the cheapest habitable square metre you can buy, and a good one typically adds around 10% to a property’s value for an outlay well below what it returns.
Think harder before converting if the garage is your only off-street parking. In streets with permit zones or genuine parking pressure, losing the drive can knock more off the value than the new room adds — a buyer who has to park two roads away notices. It is the one case where the maths can quietly reverse, so weigh it for your own street rather than in the abstract.
Extend if you need open-plan living, a proper kitchen-diner, or more bedrooms than the garage could ever hold. And remember it isn’t strictly either/or: extending beside or above the garage, or converting and extending together, is common. If there is any chance you will do both, get it drawn as one scheme so foundations and drainage are solved once rather than twice.
Tell us what you are weighing up and we will match you with up to three local specialists — some do conversions, some do extensions, plenty do both. Free, no obligation, and you decide whether to go ahead.