Extension Build Cost Per m² in 2026 (UK)
A single-storey house extension costs roughly £2,000–£2,600 per m² to build in 2026, running from about £1,600/m² for a basic shell to £3,000/m² or more for a heavily glazed, high-specification build. The per-square-metre rate is the quickest way to sanity-check a quote, but it only covers the build itself — fees, the kitchen and contingency sit on top. Here is what the 2026 rate really includes, how it splits by extension type and region, and how to turn it into a realistic total.
What builders charge per m² in 2026
The per-square-metre rate is builder shorthand for the cost of the finished build, measured against the internal floor area you gain. In 2026 a standard single-storey extension lands at £2,000–£2,600 per m² across most of the UK. Strip the finish back to a basic shell and you can see £1,600–£1,900/m²; load it with bi-folds, a lantern and a full kitchen and you are quickly at £2,800–£3,500/m².
Treat the rate as a check, not a quote. It captures foundations, structure, walls, roof, glazing to a standard spec and basic internal finish — but not professional fees, the kitchen, or the contingency every extension needs. For the full size-by-size breakdown and a live quote form, see our single-storey extension cost guide, the commercial home for extension pricing.
Typical build rate per m² by extension type — 2026
Mid-point 2026 build rate at standard specification. Glazing and finish push each type toward the top of its band.
Source: BestBuilders 2026 quote data · 519 UK towns
What the per-m² rate includes — and what it does not
The biggest mistake with a rate is assuming it is the whole job. A builder’s £/m² almost always describes the build line, including VAT, and leaves the surrounding costs for you to add:
| Cost element | Typical amount | In the per-m² rate? |
|---|---|---|
| Foundations, structure, walls, roof, standard glazing and basic finish | £2,000–£2,600/m² | Yes |
| Kitchen supply and fit | +£8,000–£25,000 | No — keep it a separate line |
| Professional fees (design, structural, planning) | +10–15% | Usually extra |
| Contingency | +10–15% | You should add it |
| Upgraded glazing (bi-folds, lantern) | +£150–£400/m² | Only if specified |
| Party wall agreements | £700–£1,500 per neighbour | Extra where a boundary is involved |
Read any headline rate as the build only, then layer fees, contingency and the kitchen on top to reach a figure you can actually budget against.
Build rate per m² by extension type
The shape of the extension changes the rate. Single-storey work is the benchmark; adding a second storey spreads the cost of foundations and roof over more floor area, so the rate per m² often falls even though the total rises. Complex corners and long structural spans push it the other way.
| Extension type | Typical rate (2026) | Why it sits there |
|---|---|---|
| Double-storey | £1,900–£2,500/m² | Shared foundations and roof over twice the floor area |
| Single-storey (rear) | £2,000–£2,600/m² | The standard benchmark for a straight rear build |
| Side return | £2,300–£2,900/m² | Narrow access, party walls and a small footprint lift the rate |
| Wrap around | £2,000–£2,800/m² | Large footprint helps, but the open corner needs steel |
| Basic shell / garage-style | £1,600–£1,900/m² | Minimal glazing and a plain internal finish |
| High-spec glazed / kitchen-led | £2,800–£3,500/m² | Bi-folds, lanterns and premium finishes |
For totals on each of these, see the rear extension, wrap around and double-storey extension cost guides.
Build rate per m² by region
Location is the other big lever. Labour and overheads in London and the South East carry a clear premium over the North and Wales — the same build spec can differ by £700/m² across the country before a single upgrade.
| Region | Typical single-storey rate (2026) | Vs UK average |
|---|---|---|
| London | £2,600–£3,300/m² | +20–30% |
| South East | £2,300–£2,800/m² | +10–15% |
| South & East England | £2,150–£2,600/m² | +5–10% |
| Midlands | £2,000–£2,400/m² | Around average |
| North West & Yorkshire | £1,850–£2,250/m² | −5–10% |
| North East & Wales | £1,700–£2,100/m² | −10–15% |
Turning the rate into a real total
The rate is only useful once you multiply it out and add the extras. Take a typical 20m² single-storey rear extension in the Midlands at £2,200/m²:
| Line | Working | Amount |
|---|---|---|
| Build | 20m² × £2,200/m² | £44,000 |
| Professional fees | ~12% | £5,300 |
| Kitchen (if applicable) | Mid-range supply & fit | £12,000 |
| Contingency | ~12% | £5,300 |
| Realistic all-in | £66,600 |
The build line was £44,000, but the cheque is closer to £67,000. That gap is exactly why a per-m² rate should start your budgeting, not end it. To price your own footprint, use our extension cost calculator or the single-storey extension calculator.
What pushes your rate above or below the band
- Glazing: bi-folds and a roof lantern are the fastest way to add £150–£400/m² over standard windows.
- Ground conditions: trees, sloping plots, high water tables or made-up ground mean deeper or engineered foundations.
- Access: a terrace with no side access, where every load is carried through the house, costs more per m² than open access.
- Structure: long open-plan spans need bigger steels and, occasionally, temporary works.
- Finish level: engineered floors, underfloor heating and premium doors quietly move a standard build up a band.
- Below the band: a simple rectangle, standard windows, open access and a plain internal finish pull you toward £1,800/m².
See the full extension cost breakdown
This page gives you the rate. For size-by-size totals, the detailed cost drivers and a quote form tuned to extensions, head to the main single-storey cost guide — the commercial home for extension pricing.
Planning permission and building regulations
Many single-storey rear extensions fall under permitted development and need no planning application, provided they stay within the size, height and boundary limits — but two-storey work, side returns and anything on a flat, listed or article 4 property usually needs full planning permission. Either way, Building Regulations approval always applies: it covers foundations, structure, insulation, drainage and fire safety, signed off by building control or an approved inspector. Our extension planning permission guide walks through the permitted development limits in detail.
Judging a quote against the per-m² rate
- Ask the builder to state both a per-m² rate and a total, so you can compare it to the £2,000–£2,600 band.
- Check whether the kitchen, fees and contingency are in or out — a suspiciously low rate usually excludes them.
- Confirm the glazing spec behind the rate; a cheap rate with premium glass on your drawings will not hold.
- Ask to see two recent extensions of a similar size you can visit or call about.
- Ask to see the builder’s public liability insurance and check the cover level yourself.
- Compare three itemised quotes on identical drawings so the rates are genuinely like-for-like.
FAQs: extension cost per m²
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