How much a house extension costs in London depends on the type, the size, the specification and the site. This guide gives realistic 2026 London rates by extension type, explains what drives the price, and shows what a good quote should cover.
Indicative house extension costs in London (2026)
| Extension type | Indicative build rate | Typical size | Planning route |
|---|---|---|---|
| Single-storey rear | £2,200 – £3,400/m² | 12–30m² | Often permitted development |
| Side return | £2,400 – £3,600/m² | 12–18m² | Often permitted development |
| Wrap-around | £2,600 – £4,000/m² | 25–40m² | Usually full planning |
| Double-storey | £2,000 – £3,000/m² | 30–60m² | Usually full planning |
Indicative 2026 London build rates. They exclude kitchen and bathroom fit-out, architect and planning fees, structural engineering, party wall and building control fees, and VAT. A free quote will be specific to your property.
What drives the cost
- The type. A double-storey extension is usually the best value per square metre, because one set of foundations and one roof serve two floors.
- The size. Fixed costs — groundworks, drainage, steelwork — are spread over more area as the extension grows, so the rate per square metre falls.
- The specification. Bifold doors, roof lanterns, underfloor heating, kitchens and bathrooms can add more than the shell itself.
- Ground conditions and access. Poor ground, drainage diversions and tight side access all add cost.
What a good quote should include
A turnkey extension quote should cover: foundations and groundworks, drainage, the structure and any steelwork, the roof, weatherproofing, windows and doors, insulation, plastering, electrics and plumbing, and building control sign-off. Kitchen, bathroom and decoration are often separate — confirm what is included.
How to compare quotes
Ask each builder to itemise inclusions and exclusions, confirm whether VAT is included, and check that structural calculations and building control are covered. A cheaper headline figure often simply excludes more.
Frequently asked questions
Is a double-storey extension cheaper per square metre?
Usually yes. The most expensive elements — foundations and roof — are shared across two floors, so the rate per square metre is lower than single-storey work, even though the total is higher.
How much does a side-return extension cost?
A side return typically costs £2,400 to £3,600 per square metre to build in London, before fit-out, fees and VAT. On a typical terrace it is often the most cost-effective way to add width.
Does the price include a new kitchen?
Usually not. Kitchen units, worktops and appliances are normally excluded from the build cost. Confirm what your quote includes.
Get a specific quote for your extension
Call +44 (0) 203 355 5313 or use our contact form. We will survey your home and give you a clear, itemised, free quote. See also our house extension service.