“How much does an extension cost?” has no single answer — it depends on size, spec, location and the state of the existing building — but you can get to a realistic ballpark quickly using per-square-metre rates. Here’s how UK extension costs break down in 2026. Treat every figure below as indicative: a proper quote always comes from a builder who’s seen the site.

The headline per-m² rates

Most extensions are priced per square metre of new floor area, then adjusted for spec and complexity. As a rough 2026 guide for England, with London and the South East at the top of each range:

Single-storey extension: roughly £2,500–£3,500 per m² for a standard build; £3,500–£4,500+ for high spec.
Double-storey extension: roughly £2,200–£3,200 per m² — adding the upper floor spreads the cost of foundations and roof, so the per-m² rate often drops slightly.
Basement (new dig): roughly £4,000–£6,000+ per m² — the most expensive option by far.

Single-storey extensions

A typical single-storey rear or side extension of around 20 m² lands somewhere around £50,000–£75,000for a standard finish, before kitchens, bathrooms and fancy glazing. A bigger open-plan kitchen-diner extension of 30–40 m² with bi-fold doors and a decent kitchen can comfortably run £90,000–£150,000+ once you include the fit-out.

Kitchens are the single biggest swing. The shell might be £60k, but the kitchen units, worktops, appliances and flooring can add anywhere from £15k to £50k+ depending on taste.

Double-storey extensions

Adding a second storey gives you far more floor area for a modest uplift in cost, because you’re reusing the same foundations and roof works. A two-storey rear extension adding, say, 40 m² across both floors often sits around £90,000–£140,000 for a standard build — frequently the best value per square metre of any extension type.

Loft and basement conversions

A loft conversion with a rear dormer and an en-suite typically costs £40,000–£70,000, with hip-to-gable and mansard conversions costing more. A basement is the premium option: converting an existing cellar might start around £1,500–£2,500 per m², but digging out a new basement with underpinning runs £4,000–£6,000+ per m² and into six figures quickly.

The extras people forget

The build cost is only part of the budget. Add:

Professional fees — architect, structural engineer, planning fees: often 10–15% of the build.
Party Wall agreements — surveyor fees if you’re near a boundary, commonly £1,000–£2,000+ per neighbour.
Building Regulations — application and inspection fees.
VAT — 20% on most extension work (new-builds differ).
Contingency — budget at least 10–15% for the surprises every project finds.

What moves the price

Location — London and the South East run materially higher than the North. Ground conditions — poor soil, trees or a high water table mean deeper foundations. Access — a terraced house with no side access means everything is barrowed through the home. Spec — structural glazing, underfloor heating and premium kitchens can double a budget. And heritage constraints — conservation areas and listed buildings push rates up and bring in specialist trades.

The honest summary

For a 2026 ballpark: single-storey around £2,500–£3,500/m², double storey a touch less per m², loft conversions £40k–£70k, basements the premium at £4,000–£6,000+/m² — then add ~15% fees, VAT and a real contingency. For contractors, the lesson is the mirror image: you can estimate a job’s value from the planning application’s floor area and proposal before you ever quote. House of Planning Service shows the floor area and an indicative property value on every planning lead, so you can size and price work before you pick up the phone.

How much does a house extension cost in 2026?