Cost calculator · 2026
London house extension cost calculator 2026.
Indicative cost ranges for every extension type in London — rear, side return, wrap-around and double-storey. Based on 200+ completed builds. Get a personalised range with your postcode and project size below.
Cost table
2026 London extension costs by type.
| Type | Cost range | Timeline |
|---|---|---|
| Single-storey rear (3m) | £45,000–£75,000 | 10–14 wks |
| Side return | £55,000–£95,000 | 12–16 wks |
| Wrap-around (L-shaped) | £85,000–£145,000 | 16–22 wks |
| Double-storey rear | £95,000–£180,000 | 18–24 wks |
| Kitchen extension (full) | £70,000–£140,000 | 14–20 wks |
Per square metre rates: £2,500–£2,900 standard finish; £3,000–£3,400 high-spec; £3,500–£3,800+ architect-led with bespoke joinery and stone. Glazing-heavy designs (sliding doors, rooflights) trend higher.
Personalised estimate
FAQ
Extension costs explained.
What is the average cost of a house extension in London 2026?+
Most London extensions cost £2,500–£3,800 per square metre fully fitted, before kitchens and bathrooms. A typical single-storey rear extension runs £45,000–£75,000, side return £55,000–£95,000, wrap-around £85,000–£145,000, double-storey £95,000–£180,000.
What does the extension cost include?+
Foundations, walls, roof, glazing, insulation, electrics, plumbing, heating extension, plaster, flooring, decoration, plus design, structural calculations, planning or LDC fees and party wall agreements. Kitchens and bathrooms quoted separately as PC sums.
Do extensions need planning permission?+
Many rear extensions are permitted development under Class A. Single-storey up to 3m on terraced/semi and 4m on detached. Prior approval extends those to 6m/8m. Double-storey, conservation areas and Article 4 zones always need planning.
How long does an extension take to build?+
Single-storey 10–14 weeks, side return 12–16, wrap-around 16–22, double-storey 18–24. Add 8–10 weeks for planning if needed, 3–4 weeks for design and tender prep.