Garage conversion vs extension: which should you build?
Same goal, two very different projects. One reuses a structure you already own. The other builds a new one. We only sell one of them — so here are the honest numbers, including where the extension wins.
Conversion or extension — which is right for you?
Convert when the space you need already exists; extend when it doesn't. An unused garage (up to ~16m² single, ~36m² double) delivers the room at roughly a third of an extension's cost per square metre, in a quarter of the time. Extend when you need more area, an open-plan kitchen, or the garage genuinely earns its keep.
How do the costs compare per square metre?
2026, this area: conversions run £850–£1,250/m²; single-storey extensions run £2,400–£3,200/m² shell-and-finish — before kitchens, large glazing or steels for open-plan openings, which push many extensions past £3,500/m² all-in. The gap is structural: the conversion inherits its foundations, walls and roof for free.
| Project | Area gained | Typical total | £/m² |
|---|---|---|---|
| Single garage conversion (dry room) | ~14m² | £12,000 – £17,500 | £850 – £1,250 |
| Double garage conversion (full) | ~28m² | £18,000 – £28,000 | £640 – £1,000 |
| Single-storey rear extension | ~15m² | £36,000 – £48,000 | £2,400 – £3,200 |
| Rear extension with kitchen & bifolds | ~20m² | £60,000 – £85,000+ | £3,000 – £4,250 |
Same-area comparison in plain terms: the room an extension delivers for £40,000, a conversion delivers for £15,000 — if you have the garage and don't need it. The itemised anatomy of conversion pricing is in the 2026 cost guide.
The full decision table
Cost is only the loudest difference. Disruption, permission burden and time diverge just as far — the conversion is a contained three-week job behind a closed garage door; the extension is months of open trench, scaffold and dust through the kitchen. The table below is the whole comparison in one place.
| Factor | Garage conversion | Extension |
|---|---|---|
| Cost per m² | £850 – £1,250 | £2,400 – £3,200+ |
| Typical total | £12,000 – £28,000 | £36,000 – £85,000+ |
| Time on site | 2 – 5 weeks | 3 – 6 months |
| Planning permission | Rarely needed — usually PD | Often needed; larger schemes always; neighbour consultation |
| Building regulations | Always — 4 inspections | Always — more stages (foundations, drainage, structure) |
| Party Wall Act | Rarely (only if altering a shared wall) | Frequently — excavation within 3m of neighbours |
| Disruption while living there | Low — work contained in the garage, house services briefly interrupted | High — open structure, weather exposure, trades through the house, often months without a kitchen |
| Garden lost | None | The extension's footprint, permanently |
| Weather risk to programme | Minimal — enclosed from day one | Real — groundwork and shell are weather-exposed |
| Value added | 5 – 15% | 10 – 20%, but against 3–4× the spend |
| Watch-outs | Covenants; parking loss on tight streets | Overdevelopment ceiling — spending past the street's price band |
When is the extension genuinely the right call?
Four honest cases. You need more than ~36m² or an open-plan kitchen-diner flowing to the garden; the garage is genuinely used (workshop, real parking on a tight street); the garage's position can't serve the room you want; or you're buying maximum long-term area and can absorb the cost and the months. Extend — no conversion spin changes it.
The kitchen case deserves emphasis because it's the most common: garages rarely sit where a kitchen wants to be, and the thing extensions do best — a wide glazed room opening onto the garden — is the thing a converted garage structurally can't. If that's the dream, save for the extension. If the need is a bedroom, office or family room, the conversion delivers it this month for a third of the money.
And the both/and route: plenty of Aylesbury households do the conversion now (£15,000, three weeks, solves the office or bedroom crisis) and the extension in five years when the kitchen fund matures. They don't compete for the same wall — done in that order, the conversion's cost is roughly the price of waiting five years for the extension to solve everything.
Conversion vs extension questions
Why is a conversion so much cheaper per square metre?
Because the expensive parts of a building — foundations, external walls, roof, weatherproofing — already exist. An extension buys all four from zero, plus groundworks and drainage moves. A conversion spends money only on the envelope upgrade and the fit-out.
Does an extension add more value than a conversion?
In absolute terms usually yes (more area added); per pound spent, almost never. A £15,000 conversion adding £25,000 returns 60%+; a £60,000 extension adding £75,000 returns ~25% — and carries the overdevelopment risk of pricing your house out of its street. The conversion maths is unpacked in the value guide.
Could I convert the garage AND extend over or behind it later?
Often, yes — a solidly built infill wall and floor don't obstruct a future extension above or beside the garage, and we build so they don't. Tell us the long-term ambition at survey and the drawings will leave the door open (sometimes literally, with a lintel positioned for a future opening).
Is a conservatory or garden room a middle option?
Price-wise yes, performance-wise no: conservatories are seasonal rooms, and garden pods sit outside your insulated envelope and depreciate. A converted garage is real, permanent, year-round floor area — the comparison is on the home office page where it comes up most.
How do I actually decide?
Write down the room you need and its size. Under ~16m² and the garage is idle: convert. Kitchen-diner dreams or 20m²+: extend, or phase it. Genuinely torn: our free survey prices the conversion and tells you honestly if your brief is an extension brief — book one.