Planning an extension and wondering what it will really cost? This guide brings together published 2026 figures for London, explains what is and is not included in a “per square metre” price, and shows the extra costs that catch people out.
Figures are published 2026 ranges from HomeOwners Alliance, Homebuilding & Renovating and Bookabuilder, not Arkivon quotes. Every house is different, so use them for budgeting and get a fixed written quote.
House extension cost per m² in London
| Type | Typical London build cost per m² | Example total |
|---|---|---|
| Single storey rear extension | £2,300 to £3,400 | 30m²: about £70,000 to £100,000 |
| Double storey extension | £1,900 to £3,000 | 50m²: about £95,000 to £150,000 |
| Kitchen extension (side return or rear) | Build as single storey, plus the kitchen | 25m² plus kitchen: about £75,000 to £120,000 |
Per m² rates usually cover the shell, structure and basic finishes. They usually exclude VAT, professional fees, the kitchen itself, high-end glazing and landscaping.
Single storey extension cost
Single storey rear and side-return extensions are the most popular in London, especially on Victorian and Edwardian terraces. They cost more per m² than double storey because the foundations and roof are spread over less floor area. Big sliding or bi-fold doors and roof glazing can add a lot: specify them early so your quote is realistic.
Double storey extension cost
A second storey adds bedrooms or a bathroom for a lower cost per m², because the same foundations and roof serve twice the floor area. Total cost is higher, and double storey extensions are more likely to need full planning permission.
Kitchen extension cost
A kitchen extension is really two budgets: the building work and the kitchen. Kitchens range from around £10,000 for a simple fitted kitchen to £40,000 or more for bespoke units, stone worktops and premium appliances. Plan the layout before you build, so drainage, gas and electrics go in the right place first time.
Extra costs to budget for
| Item | Typical allowance |
|---|---|
| VAT | 20% on most building work |
| Architect and planning drawings | Several thousand pounds |
| Structural engineer | Often £1,000 to £3,000 |
| Planning or lawful development certificate fees | Set by the government |
| Building control | Council or approved inspector fee |
| Party wall surveyor (terraces and semis) | Per neighbour, if they appoint one |
| Contingency | 10 to 15% of the build |
How to keep extension costs down
- Keep the shape simple: corners, steps and complex roofs add cost.
- Use standard door and window sizes.
- Fix the specification before building starts; changes mid-build are expensive.
- Check permitted development first, which can save time on planning.
- Get like-for-like quotes that list what is included.
Frequently asked questions
How much is a 3m x 4m extension in London?
At 12m², a simple single storey extension would typically fall around £30,000 to £45,000 before VAT and fees, using published London rates.
Is a double storey extension worth it?
Often yes: the second storey costs less per m² and adds bedrooms, which add the most value in London.
Do I need planning permission?
Many single storey rear extensions fall under permitted development; double storey and side extensions more often need permission.
See our house extension service, recent projects and our loft conversion cost guide.
Want a fixed price for your extension? Book a free site visit or call 07754 690740.
House extension builders in: Barnet, Golders Green, Harrow, Hampstead, Mill Hill, Crouch End, Belsize Park, Muswell Hill. See all areas we cover.
Related local pages: house extensions in Richmond, house extensions in Highgate.
Omar runs Arkivon Group Ltd, a London building company based in NW10 that handles extensions, loft conversions, roofing and full renovations across North and West London. Ask Omar a question.