Quote check · September 2026
Is my extension quote fair?
In September 2026 a 25 m² single storey extension typically costs £63,000 to £78,000 including VAT to build outside London, and £85,000 to £105,000 in London. With design, a structural engineer, building control and a 15% contingency, a sensible all-in budget is £78,500 to £102,000. A quote inside the build range for your region, for the same scope, is a normal market price.
What extensions cost in 2026
These are the figures behind our calculator for a typical house and a mid-range finish. The build cost is what a builder's quote usually covers; the right-hand column is the whole budget.
| Extension type | Build cost, UK average | Build cost, London | All in, UK average, with fees and 15% contingency |
|---|---|---|---|
| Single-storey rear (25 m²) | £63,000 to £78,000 | £85,000 to £105,000 | £78,500 to £102,000 |
| Double-storey (40 m²) | £91,000 to £115,000 | £123,000 to £156,000 | £112,000 to £147,000 |
| Wraparound or side return (30 m²) | £83,000 to £104,000 | £112,000 to £141,000 | £101,000 to £132,000 |
Single storey prices by region
Compare your quote with the row for where you live, or use the quote checker with your postcode.
| Region | Build cost | All in with fees and 15% contingency |
|---|---|---|
| UK average (outside London) | £63,000 to £78,000 | £78,500 to £102,000 |
| London | £85,000 to £105,000 | £104,000 to £133,000 |
| South East | £72,500 to £89,500 | £89,000 to £115,000 |
| East of England | £67,500 to £83,500 | £83,500 to £108,000 |
| South West | £65,500 to £81,000 | £81,000 to £105,000 |
| West Midlands | £61,000 to £75,500 | £76,000 to £99,000 |
| East Midlands | £60,000 to £74,000 | £74,500 to £97,000 |
| Scotland | £60,000 to £74,000 | £74,500 to £97,000 |
| North West | £58,500 to £72,500 | £73,000 to £95,000 |
| Yorkshire and the Humber | £58,000 to £72,000 | £72,500 to £94,500 |
| Wales | £56,500 to £70,000 | £71,000 to £92,500 |
| North East | £55,500 to £68,500 | £69,500 to £90,500 |
| Northern Ireland | £51,500 to £64,000 | £65,000 to £85,500 |
What extension quotes often leave out
- The kitchen itself, worktops and appliances, which are often priced separately
- Bifold or sliding doors and roof lanterns, sometimes as provisional sums
- Flooring, decoration and making good the existing rooms
- Drainage work: diverting a drain, or a build-over agreement with the water company
- Steel beams and padstones where the back wall is opened up
- Heating: new radiators or underfloor heating, and whether the boiler can cope
- Building control and the structural engineer, if the builder expects you to appoint them
- A party wall surveyor if you share a wall or dig near a neighbour's foundations
See every extra with typical prices in the hidden costs of an extension.
Signs an extension quote is too high or too low
Possibly too high: well above the range for your region with no clear reason. Good reasons include poor access, deep foundations, steelwork, glazing-heavy designs and a high specification.
Possibly too low: long exclusion lists, large provisional sums, no mention of building control or structural calculations, no written specification, or a large payment up front.
Questions to ask before you sign
- Is the price fixed, and what could change it?
- What is excluded, and what are the provisional sums?
- Who appoints building control and the structural engineer?
- What is the payment schedule, and what must be finished before each payment?
- What insurance and guarantee come with the work?
- When can you start, and how long will it take?