How much does a house extension cost? How long does it take to build?
Build costs are quoted per square metre and vary hugely with shape, specification and site; the 2020 market guidance below is now a floor rather than a current quote. The statutory costs are fixed and knowable: a £548 planning fee from April 2026, plus building regulations charges, and 20% VAT unless your project is a new build or a qualifying conversion.
House extension cost guide
As a rule of thumb, extension prices can start from £1,100 per square metre (plus VAT) and rise well over £2,600 per square metre (plus VAT).
The final figure depends on quality, complexity, specification, shape, size, number of storeys, drainage, layout, site conditions and location. Narrow, small extensions tend to cost more per square metre than square, large ones, and prices in London are higher than in other areas.
You should also factor in kitchen elements and whether you will be able to stay in the property while the work is carried out.
Additional costs to consider
On top of the building costs, there are several other expenses to budget for:
- Drawings and structural calculations for planning and building regulations (from around £800)
- Party wall agreement (around £1,000 if an agreed surveyor is used)
- Insurance cost
- Internal alterations, rewiring, asbestos removal, a new boiler system and decoration
Example 1: single-storey extension
An 18 square metre (6m x 3m) single-storey rear extension with bi-fold doors, a roof lantern and a flat roof would cost around £27,000 plus VAT (20%), plus kitchen cabinets and sanitaryware.
This typically takes around 2 months to build, depending on site and weather conditions.
Example 2: double-storey extension
A 36 square metre (6m x 3m) double-storey rear extension would cost around £50,000 plus VAT (20%), plus kitchen cabinets and sanitaryware.
This typically takes around 3 months to build, depending on site and weather conditions.
Reading these 2020 figures in 2026
The per-square-metre figures above were market guidance in 2020 and they now understate what a builder will quote. Construction output prices have risen every year since, and the ONS put the annual rate of construction output price growth at 0.8% in the 12 months to March 2026, so the increase has slowed but not reversed. Treat the 2020 rates as a floor and the shape of the advice, not as a current quote.
What has not changed is which decisions move the number. Narrow extensions still cost more per square metre than wide ones, glazing and roof lanterns are still where budgets go, and drainage or a difficult access is still what turns a predictable job into an expensive one.
The statutory costs, which are fixed and knowable
Unlike build costs, the official fees are published and current. These are the ones you can budget exactly:
- Planning application fee, from 1 April 2026: £548 to enlarge, improve or alter a single dwellinghouse; £1,083 for two or more dwellinghouses; £272 for works within the curtilage such as an outbuilding, fence or wall. Indexed each April.
- Building regulations charges: set by each local authority or approved inspector, so they vary by area rather than being national.
- Party wall surveyor, where the work is notifiable: usually paid by the building owner doing the work.
- Build over agreement, where you are within 3m of a public sewer: from £130 for the smallest sewers with Thames Water, plus a CCTV survey.
VAT: 20%, unless your project is one of the exceptions
A straightforward extension to an existing house is standard-rated at 20%, and that is most projects. But three exceptions are worth knowing because they are worth far more than any saving on drawings.
Building a new dwelling can be zero-rated where it meets the qualifying conditions. Converting a non-residential building into a dwelling is reduced-rated at 5%. And renovating or altering a dwelling that has been empty for the past two years is also reduced-rated at 5%. If your project is a conversion or you are bringing a long-empty property back into use, raise VAT with your builder and accountant before the quotes come in, not after.
Get multiple quotes
Because so many factors affect the final price and timeline, it is important to get multiple quotes for house extensions before you commit. The per-square-metre figures here are general market guidance from 2020, not AVA's fee, and should be treated as a starting point.