A house extension in London usually costs somewhere between £2,500 and £5,000 per square metre of new floor area, which puts a typical single-storey rear extension at roughly £55,000 to £110,000. The range is wide because “an extension” covers everything from a simple box off the back to a wrap-around with a new kitchen and structural glazing.
These are indicative 2026 figures for guidance. Get a fixed price after a site visit before you commit.
Cost by extension type
- Single-storey rear extension – £55,000 to £110,000. The most common project. Around £2,500-£3,500/m² for a good standard finish, £3,500-£5,000+/m² with premium glazing and a fitted kitchen.
- Side return extension – £40,000 to £70,000+. Filling the narrow alley beside a Victorian or Edwardian terrace. Smaller floor area but a big change to light and layout.
- Wrap-around (side + rear) – £70,000 to £130,000+. Combines both and usually delivers the biggest open-plan kitchen-diner.
- Double-storey rear extension – roughly 50-60% more than single-storey for close to double the space, because the roof and foundations are shared. Better value per square metre.
- Basement conversion – £250,000+. The most expensive way to add space in London; only makes sense where footprint is fully used.
Costs people forget to budget
- Design and drawings: architect or designer £2,000-£6,000.
- Structural engineer: £800-£2,000 for calculations.
- Planning / prior approval / lawful development fees: a few hundred pounds.
- Building control: £500-£1,000.
- Party wall surveyors: £1,000-£2,000+ per surveyor if neighbours do not consent.
- Kitchen: £8,000-£25,000 depending on units and appliances.
- Glazing: bi-fold or sliding doors £3,000-£10,000; a structural glass roof or lantern adds more.
- Decoration, flooring and making good the rooms the extension connects to.
- VAT – domestic building work is standard-rated unless a specific relief applies.
What changes the price most
- Ground conditions – drains to divert, tree roots, a shared sewer needing a build-over agreement, or deeper foundations.
- How much glazing, and whether it is structural (no visible frame).
- Kitchen and bathroom specification.
- Roof type – a simple flat roof is cheaper than a pitched or lantern roof.
- Access to the rear of the property for materials and muck-away.
Arkivon Group builds house extensions across London on a fixed price, with design, planning and build under one roof. Book a free site visit for a firm figure for your home, or see recent projects.