House extension cost UK: the full 2026 price guide
Almost every extension project starts with the same question: how much is this going to cost? The honest answer is a range, not a number — but it's a range you can narrow a long way once you know the four things that move the price: the type of extension, its floor area, where you are in the UK, and the specification you're building to. This guide puts real 2026 figures against all four, then shows how the hidden costs — fees, VAT and contingency — turn a build quote into a project budget.
The short answer
In 2026, a standard single-storey rear extension costs roughly £1,900 to £2,600 per square metre to build, before VAT, professional fees and contingency. So a typical 20 m² extension is around £40,000–£60,000 for the build alone, and closer to £55,000–£85,000 all-in. Go basic and the rate drops toward £1,700/m²; go premium — structural glazing, vaulted ceiling, underfloor heating — and it climbs to £2,800–£3,500/m².
Cost per square metre by specification
Specification is the single biggest lever on the rate. "Basic" means a simple rectangular plan, a flat or mono-pitch roof, modest glazing and standard finishes. "Premium" means large sliding or bifold doors, roof lights or a glazed roof, a bespoke kitchen and high-end flooring.
| Specification | Single-storey £/m² | What you get |
|---|---|---|
| Basic | £1,700 – £2,000 | Simple box, flat roof, standard doors and finishes |
| Standard | £1,900 – £2,600 | Pitched or lantern roof, French or patio doors, mid-range kitchen |
| Premium | £2,800 – £3,500 | Structural glazing, glazed roof, bespoke kitchen, underfloor heating |
Total cost by size
These are single-storey, standard-specification guide figures. The "all-in" column adds professional fees, VAT on the build and a 12% contingency.
| Floor area | Build cost (ex-VAT) | All-in guide |
|---|---|---|
| 15 m² (small) | £28,000 – £39,000 | £40,000 – £55,000 |
| 20 m² (3m × 6.5m) | £38,000 – £52,000 | £55,000 – £75,000 |
| 25 m² | £48,000 – £65,000 | £68,000 – £92,000 |
| 30 m² (medium) | £57,000 – £78,000 | £80,000 – £110,000 |
| 50 m² (large) | £95,000 – £130,000 | £130,000 – £180,000 |
Cost by extension type
Two extensions of the same floor area can cost very different amounts because complexity — structural openings, roof junctions, foundations — varies by type.
| Type | Typical £/m² | Notes |
|---|---|---|
| Single-storey rear | £1,900 – £2,600 | The baseline; cheapest in total |
| Side return | £1,900 – £2,200 | Small footprint, big impact on a terrace |
| Kitchen extension | £1,900 – £2,600 + £8k–£15k kitchen | Fit-out adds a lot on top of the shell |
| Double / two-storey | £2,000 – £3,000 | More in total, less per m² of new space |
| Wrap-around | £2,400 – £3,000+ | Side + rear combined; most roof detailing |
Where you are in the UK
Labour and, to a lesser extent, materials cost more in London and the South East and less in the North and Wales. Apply this to whichever rate you've landed on above.
| Region | Vs UK average |
|---|---|
| London | +25% to +40% |
| South East | +10% to +25% |
| South West | +5% to +15% |
| Midlands | around average |
| North England | −5% to −10% |
| Scotland | around average (cities higher) |
| Wales | −10% to −15% |
| Northern Ireland | below average |
The costs that aren't in the £/m²
A builder's quote is the build. The project costs more. Budget separately for:
| Item | Typical 2026 cost |
|---|---|
| Architect / designer | £1,500 – £3,000, or 8–15% of build cost |
| Structural engineer | £400 – £2,000 |
| Planning application (England) | £258 – £548 |
| Building Regulations | £400 – £1,000 |
| Party wall surveyor (if needed) | £700 – £2,000 |
| VAT on the build | 20% — extensions are standard-rated |
| Contingency | 10 – 15% of build cost |
Together, fees and admin usually add £3,000–£6,500. VAT is the big one people forget: most builder quotes are stated excluding VAT, so a £50,000 build is really £60,000.
Not every extension needs full planning permission — many fall under Permitted Development — but building regulations approval is a near-universal requirement whichever planning route you take.
How long does it take?
A single-storey extension is typically 10–14 weeks on site once work starts. A double-storey extension runs 12–20 weeks. Before any of that, design, planning and Building Regulations sign-off commonly take two to four months, so a spring start usually means engaging a designer the previous autumn.
Does an extension add value?
Usually yes, but rarely pound-for-pound in the short term. A well-judged extension returns roughly £1.20–£1.50 for every £1 spent and can lift a home's value by 10–20% — as long as it doesn't push the house past the ceiling price for the street. We cover the numbers by room type in a separate guide on whether an extension adds value.
How to budget in practice
- Use the calculator to get a type/region/spec-adjusted mid-point.
- Add the fees table above and 20% VAT on the build.
- Add a 12–15% contingency on top of that.
- Get three like-for-like quotes covering an identical specification — not "an extension", but the same drawings, the same doors, the same roof.
- Treat the lowest quote with caution if it's a long way below the other two.
Frequently asked questions
How much does a house extension cost in the UK in 2026?
A standard single-storey rear extension is about £1,900–£2,600 per m² to build. A 20 m² extension is roughly £40,000–£60,000 for the build, or £55,000–£85,000 all-in with fees, VAT and contingency. Double-storey work is about £2,000–£3,000 per m² of total floor area.
What is the average cost of an extension per square metre?
£1,700–£2,000/m² basic, £1,900–£2,600/m² standard, £2,800–£3,500/m² premium. London and the South East add 15–40%.
Why are extension cost calculators so inaccurate?
Because the rate swings with spec, region, roof design, ground and access — a single national average can be 40% out. A calculator that lets you set type, region and spec gets much closer, but only local quotes are firm.
How much should I add for a contingency?
10–15% of the build cost. Drainage diversions, poor ground and hidden structural issues are common once walls come down.
Does an extension need planning permission?
Many single-storey rear extensions are covered by Permitted Development within size and height limits. Building Regulations approval is separate and applies to nearly every extension.
A quick note
These are indicative 2026 UK figures gathered from published cost guides. Real costs depend on your drawings, site and local market — always get multiple like-for-like quotes before committing. See our full disclaimer.