As a rough guide, a single-storey rear extension in West London runs about £2,500 to £3,500 per square metre, so a typical 20 to 25 m² project lands somewhere between £55,000 and £90,000 before VAT and fit-out. The real number depends on size, structure, finish and access, which is why no honest builder quotes one off the back of a phone call.
What actually moves the price?
Four things, mostly. Size and the number of storeys come first: a double-storey extension reuses the same foundations and roof, so the cost per square metre usually falls even as the total goes up. Structure is next, so steel for a knock-through, deeper foundations on the London Clay under much of Ealing and West London, or underpinning all add cost. Then there’s specification, where the gap between a standard finish and a high-end one (slimline glazing, rooflights, proper joinery) can be half the build cost again. And finally the kitchen, because most rear extensions are really a new kitchen, and that’s the line people routinely leave out of their sums.
Indicative 2026 costs
| Extension type | Guide £/m² | Typical project total |
|---|---|---|
| Single-storey rear | £2,500–£3,500 | £55k–£90k (≈20–25 m²) |
| Side-return | — | £55k–£95k |
| Double-storey | £2,200–£3,200 | £90k–£150k+ |
| Wrap-around | — | £80k–£140k+ |
Treat these as starting points, not quotes. Prime-central and heritage addresses sit above them; a simple suburban build can come in below.
The costs people forget
The build figure is not the whole project. Add design and a structural engineer, planning fees, and Building Regulations sign-off. If you share a wall with a neighbour, which covers most terraces and semis, the Party Wall etc. Act 1996 means serving notice and usually appointing a surveyor. Add VAT at 20%, which too many quotes quietly omit, plus flooring and decoration, and keep about 10% back as contingency. Something always shows up once the floor is lifted.
Do you need planning permission?
Often you don’t. Many single-storey rear extensions count as permitted development, within limits: a rear extension generally cannot project more than three metres past the original rear wall (four metres for a detached house), or stand more than four metres tall. Two things trip people up. Some streets have had those rights removed by an Article 4 direction, so a full application is needed even for modest work. And conservation areas and listed buildings always need consent. We handle the drawings and the application either way; the house extensions page walks through it.
Planning something in Ealing, Richmond or Chiswick? Get a fixed quote and we’ll survey and price it properly.
