Extensions
House extensions: costs, planning and quotes
Costs worked out by a quantity surveyor, including the design fees and contingency most price guides leave out.
- Typical all-in cost
- £44,400 to £87,500
- Rates checked
- November 2026
- Free calculator
- Price your project
- Your details
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What it costs
National baseline prices for a standard finish, including VAT. London is around 25% higher; the North and Northern Ireland lower.
Price your own project| Project | Building work | Fees | Contingency | All-in |
|---|---|---|---|---|
| Single-storey rear extension, 15 m² | £36,100 | £4,300 | £4,000 | £44,400 |
| Single-storey rear extension, 25 m² | £60,000 | £7,200 | £6,700 | £73,900 |
| Two-storey extension, 36 m² over both floors | £71,000 | £8,500 | £8,000 | £87,500 |
What affects the price
- Size, and whether it is one storey or two
- Openings knocked through into the house, and the steel beams they need
- A new kitchen or bathroom inside the extension
- Glazing: bifold doors and roof lanterns add thousands
- Ground conditions, which set how deep the foundations go
- Access: narrow side passages slow every delivery
Do you need planning permission?
Check my projectThese are the rules for houses in England. Flats, listed buildings and conservation areas have tighter rules, and Wales, Scotland and Northern Ireland differ. Check with your council before you build.
- A single-storey rear extension can usually go ahead without planning permission if it is no more than 3m deep on an attached house or 4m on a detached house, and no more than 4m high.
- A larger single-storey extension, up to 6m on an attached house or 8m on a detached house, needs prior approval from the council, which is quicker than a full application.
- A two-storey rear extension is permitted development only if it is no more than 3m deep and at least 7m from the rear boundary; otherwise it needs planning permission. In conservation areas and other protected areas, side and two-storey extensions always need planning permission.
- Every extension needs building regulations approval, and work on or near a shared wall may need a party wall agreement.
Extension guides
Guides for this topic are being written.
Getting quotes
How to find a good builder
A fair price starts with comparing the same job across several firms.
- Ask for three quotes from the same drawings. Without a shared specification, each firm prices a different job.
- Check each firm before you invite it. Look it up at Companies House, ask to see its insurance certificate, and speak to a recent customer. TrustMark, the government-endorsed scheme, lists registered firms.
- Compare what is included, not just the total. Our quote checker shows how a quote compares with our estimate and what it leaves out.