How the calculator works
Building work is a price per square metre times the floor area you add. The standard rate is the middle half of every rate five UK cost guides publish for that storey. No government series prices extensions, so we show the guides' figures side by side in our house extension cost guide rather than pick one.
| Spec | Single storey, per m² | Double storey, per m² |
|---|---|---|
| Basic | £1,250 to £1,800 | £1,050 to £1,600 |
| Standard | £1,700 to £2,400 | £1,450 to £2,200 |
| High | £1,900 to £2,700 | £1,750 to £2,650 |
The data behind this table
The lower and upper quartile of every rate MyJobQuote, HomeOwners Alliance (Checkatrade's figures), the Federation of Master Builders, Rated People and Bark publish for each storey, rounded to the nearest £50 and read on 11 November 2026. Rated People gives totals, so we divided them by floor area. Basic and high keep MyJobQuote's spacing, the only guide that prices by spec.
Size is straight multiplication, the way HomeOwners Alliance, Bark and MyJobQuote price it. Rated People's totals fall per square metre as size grows, so a very small extension may cost more than this and a very large one less. Extras and fees are the middle half of the published figures for each item, and the contingency is 10%. Planning follows the Planning Portal's limits for England, and the fees are GOV.UK's from 1 April 2026. Heating uses our heat model. The working is all in js/extension-cost.js.
We leave out what we cannot source: regional prices, bifold doors, moving drains, decorating, furniture and VAT where a guide's price excludes it. For the planning limits in full and building regulations, read the planning section of the guide.
Frequently asked questions
How accurate is this extension cost calculator?
It gives the range a typical job of your size and spec falls in, not a quote. The building work is the middle half of the rates five UK cost guides publish, and those guides disagree widely, so a local builder can quote outside it. Get at least three itemised quotes for the same drawings.
What is not included in the estimate?
Decorating, furniture, bifold doors, moving drains, unusual foundations and VAT where a guide's price leaves it out. Some builders include drawings, building control and scaffolding in their price and some do not, which is why the calculator shows the fees as separate lines.
Why is there no adjustment for my region?
We only adjust prices by region using official pay data, and we have not yet built that for building trades. The guides that mention region all say London and the South East cost more: MyJobQuote puts labour there 10 to 25% above the national average.
How is the heating cost worked out?
From our heat model: the median gas use per square metre for a home like yours, scaled to the level our model measures for homes rated EPC band A or B, because new walls, roof and floor must meet current insulation standards. It is priced at the October to December 2026 Ofgem price cap for your heating.
Data sources
- MyJobQuote, House Extension Cost (updated 19 October 2026): rates by storey and spec, fees, shower room and time on site.
- HomeOwners Alliance, House extension costs (undated): Checkatrade's rates, kitchen, fees and the 10% contingency.
- Federation of Master Builders, Two-storey extension costs (13 January 2026): rates for single and double storey extensions.
- Rated People, How much does a house extension cost in 2026?: totals by size and type, fees and time on site.
- Bark, House Extension Costs 2025 (undated): rate per square metre and architect fee.
- Planning Portal, extensions planning permission: permitted development limits in England.
- GOV.UK, fees for planning applications in England from 1 April 2026: £548 householder application, £249 prior approval, and a lawful development certificate at half the application fee.
- Approved Document L, Volume 1, 2021 edition: insulation limits (Table 4.2) and the glazing limit for extensions (paragraph 10.7).
- Heating from our heat model (DESNZ NEED 2026) at the Ofgem price cap for October to December 2026. All guides read on 11 November 2026.