How Much Does an Extension Cost in the UK? (2026 Guide)

The starting point is a build rate per square metre of new floor area. For a single-storey extension in 2026, budget roughly £2,000/m² for a basic finish, £2,500/m² for a standard mid-market spec, and £3,200/m² or more for a high specification. Those are national averages before you adjust for where you live and before extras.

So a 25 m² single-storey rear extension to a standard spec starts around £62,500 as a bare build cost. That figure is the shell and the basic finish. It doesn't yet include a kitchen, fees, or the 20% VAT that applies to domestic extension work. Add those and the same job comfortably passes £80,000.

Basic, standard and high aren't marketing labels, they're real differences in what you get. Basic means a blockwork shell, standard plaster, functional sanitaryware and modest glazing. Standard is what most people build: cavity insulation to current Part L, decent doors, tiled or luxury vinyl floors, a mid-range kitchen. High means large glazing, underfloor heating, bespoke joinery and premium finishes throughout.

Here's the counterintuitive bit. A double-storey extension costs more in total than a single-storey one, but usually less per square metre. Budget around £1,700–£2,700/m² of total floor area across both storeys.

The reason is simple once you see it. The two most expensive parts of any extension are the groundwork below and the roof above. A double-storey extension shares one set of foundations and one roof across two floors, so those big fixed costs get spread over twice the floor area. You're buying a second storey at close to the cost of the walls and floor alone.

That's why "go up as well as out" is often the better-value move if you need the space and the structure allows it. A two-storey extension rarely doubles the bill of a single-storey one of the same footprint. It tends to add somewhere around 50 to 60 percent more cost for 100 percent more floor.

Building costs aren't uniform across the country. The Building Cost Information Service (BCIS), the RICS-owned body that tracks UK construction costs, publishes regional variation that any decent estimate should reflect. London and the South East run well above the national average. The North East, Wales, Scotland and Northern Ireland run below it.

The gap is bigger than most people expect. Between a London job and a North East one, the same 25 m² extension can differ by £15,000 to £20,000 on location alone, before a single design choice is made. Labour is the main driver: a bricklayer's day rate in the South East is a different number from one in County Durham.

This is why a headline figure you saw on a national website is close to useless for your actual budget. It's an average of high-cost and low-cost regions blended together. Price to your region, not to the middle of the country.

The build rate is the number people fixate on. The costs that actually blow budgets are the ones underneath it.

Take a real-shaped project. Aisha is building a 25 m² single-storey rear kitchen extension in Leeds, in Yorkshire and the Humber, to a standard spec. She wants a new kitchen and bi-fold doors, and she's including professional and statutory fees.

Most cost guides lead with a per-square-metre figure, and it's the least useful number in your whole budget. Per-m² rates make sense for the shell, but the shell is the part that behaves predictably. The costs that decide whether you're happy or broke at the end, the kitchen, the glazing, the fees, the groundworks you couldn't see, are largely fixed and barely scale with area. Two 25 m² extensions next door to each other can differ by £40,000 purely on fit-out and ground conditions, while sharing an identical per-m² shell rate. Budget the fixed costs as line items, not as a multiplier.

In practice, most UK builders price a job off the drawings and a site visit, not off a per-m² rule, and they load the number for access, muck-away and anything they can't see below ground. A typical 2026 quote for a standard rear extension in the Midlands might come in around £2,600 to £3,000/m² all-in once fees and a decent kitchen are folded in, and the honest ones will tell you the groundworks line is the one most likely to move. Get two or three quotes on the same drawings, and treat the cheapest one with the most suspicion, not the least. When a number comes in well under the others, it usually means something's been left out that you'll pay for later.

Start with a realistic estimate from the Extension Cost Calculator UK so you know the ballpark before you talk to anyone. Then get drawings done, because no builder can price accurately without them. Send the same drawings to two or three builders, ask each to itemise the groundworks, structural steel and any provisional sums, and check what's excluded. Materials from a merchant like Travis Perkins or Jewson will sit inside the builder's price, but knowing rough material costs helps you sense-check where the money's going.