Garage Extension Costs in 2026: Building Above or Beside a UK Garage

Garage extension costs in 2026

There are two ways to extend a garage: build a room above it, or build out beside it. Above is the value play — the slab, walls and drainage already exist, so a bedroom or office over the garage runs £1,400–£2,000 per m². That puts a 15m² room at roughly £21,000–£30,000 and an 18m² room at £25,000–£36,000, before professional fees.

Building beside the garage is a conventional project at the standard rates in our single storey extension guide: £1,900–£2,600 per m² at a standard spec, with the full element-by-element picture in the cost per m² breakdown. Add 20–30% in London and the South East either way, and note that builder quotes to homeowners normally include VAT.

The structural survey: the gate every garage extension must pass

Attached garages are usually built down to a price: shallow strip foundations, half-brick walls and a wide door opening, all sized for a lightweight roof and a car — not for a floor structure, a storey of masonry and a new roof. Whether yours can carry the load is not something any builder can judge by eye.

So the first cheque goes to a structural engineer: £600–£1,400 for a survey, normally including a trial pit dug against the footings. There are two outcomes. Pass, and you proceed at the attractive £1,400–£2,000 per m² rate. Fail, and underpinning adds £5,000–£15,000 — and occasionally a wall needs rebuilding too, at which point it is worth pricing a fresh build instead. Never accept a quote that has not answered this question in writing.

Garage extension costs (2026)

Costs assume a straightforward attached garage outside London; add 20–30% in London and the South East, plus 10–15% professional fees on any build route.

Building beside the garage instead

If the foundations are marginal, or what you actually need is ground-floor living space, extending beside or behind the garage is the straightforward route. It is priced like any single storey extension — £1,900–£2,600 per m² at standard spec, so around £30,000–£40,000 for 15m² — and takes 8–12 weeks on site.

Watch for the garage-specific extras: relocating the garage door or losing it altogether, and moving the services that so often live in garages — consumer units, meters and stopcocks all cost money to shift. Browse local extension builders who have handled garage plots before, and get the services question priced explicitly.

Planning permission for garage extensions

Adding a storey above a garage usually needs a full planning application. Permitted development for side additions covers single storey only — capped at half the width of the original house and 4m high — and the two-storey allowance is limited to rear extensions kept at least 7m from the rear boundary, which rarely helps a side garage. Conservation areas and listed buildings are excluded from permitted development entirely.

Check your address on the Planning Portal before commissioning drawings, and remember building regulations always apply: the inspector will want the engineer evidence on the foundations before signing off a new floor over the garage.

What drives garage extension costs up or down

Beyond the headline rate, six things decide where your project lands:

• Foundation condition: the pass-or-underpin question is a £5,000–£15,000 swing on its own
• The roof junction: tying a new pitched roof into the main house is skilled work — a dormer-style design can simplify the join on some rooflines
• Matching materials: sourcing bricks and tiles that sit comfortably against thirty-year-old weathered ones takes time and money
• Services: meters, consumer units and boilers that live in the garage may all need relocating
• A bathroom above: adds £4,000–£8,000, with pipework rising through the garage
• Stairs and access: connecting the new room to the landing can steal space from an existing bedroom

Run your own numbers through the extension cost calculator before you invite quotes, so you can spot an outlier instantly.

FAQs: Garage Extension Costs