How much does a London
extension cost in 2026?
The honest number.
Side-return, rear, wraparound, double-storey or basement — the real 2026 figures we use on client quotes. Built into a 60-second calculator so you can price your project without a callback.
Pick your shape. See the price.
All figures inclusive of design, planning, build, contingency and VAT. Tighter numbers for your postcode and spec live inside the calculator.
The breakdown most builders won't show you.
Worked example for a typical £200k single-storey rear extension in a SW6 or W4 postcode. Every project gets a line-by-line bill of quantities, not a one-page lump-sum quote.
Permitted development or planning permission?
It changes your timeline by 8 weeks.
Permitted development (PD)
For single-storey rears up to 3m (terrace/semi) or 4m (detached), within height limits, no balconies, materials matching the existing house. Excluded if you’re in a conservation area or under an Article 4 direction.
Best for: simple kitchen-diner rears outside conservation areas.
Full planning permission
Required for wraparounds, double-storeys, contemporary glazing, anything over PD limits, and most conservation areas. Slower but unlocks much better design — especially for Victorian terraces with side returns.
Best for: wraparounds, double-storeys, anything in SW3/SW6/W11 conservation zones.
Party wall awards
Required by law if you’re excavating within 3m of a shared wall, building on the boundary line, or affecting a party structure. Each neighbour usually appoints their own surveyor. Costs typically £1,000–£3,000 per agreement.
Required for: most rear extensions, all basement digs, almost every London terrace project.
The 2026 London Extension Cost Guide.
Twenty-four pages built from real client projects across SW6, SW3, W11, SW11 and W4. Sent to your inbox the moment you submit.
- ✓Side-return, rear, wraparound & double-storey costs side by side
- ✓Full £200k extension cost breakdown, line by line
- ✓Borough multipliers — Zone 1 vs Zone 2 vs Zone 3
- ✓Planning permission vs PD — the real timeline difference
- ✓Party wall awards & build-over agreements explained
- ✓The 7 hidden costs that catch homeowners out
Send me the guide.
What homeowners ask us before they book.
How much does a house extension cost in London in 2026?
Typical 2026 London ranges (inc. design, build, contingency & VAT): side-return extensions £90k–£160k; single-storey rear £110k–£210k; wraparound £170k–£320k; double-storey rear £180k–£380k; basement dig-outs £250k–£750k. Postcode (Zone 1 vs Zone 3), specification tier and structural complexity move you within the range.
What's the cost per m² for a London extension in 2026?
Ground-floor extensions typically run £3,300–£5,000 per m² turnkey in central and inner-west London. Double-storey extensions are slightly cheaper per m² (£3,100–£4,400) because you share foundations and roof across two floors. Basements sit at £5,500–£9,500 per m² because of underpinning and waterproofing.
Do I need planning permission for my extension?
Single-storey rear extensions up to 3m (semi-detached/terrace) or 4m (detached) on the original house often fall under permitted development (PD) rights — no planning needed. Side returns, wraparounds, double-storeys and anything in a conservation area or under Article 4 direction usually require full planning permission. We assess your address before quoting.
How long does a London extension take from first call to handover?
Allow 8–12 weeks for design and planning, then 16–22 weeks on site for a single-storey, 22–30 weeks for a double-storey or wraparound, and 28–40 weeks for a basement. Our programme is written into the contract — late completion is our problem, not yours.
What hidden costs catch London homeowners out?
The top seven: party wall awards (£1,000–£3,000 per neighbour), build-over agreements with Thames Water (£300–£1,500), planning fees and conservation consultations, structural engineer fees, drainage surveys, asbestos surveys on pre-2000 properties, and VAT (which is 20% on top of the build cost). All of these are itemised inside our quote — no surprises.
Should I use permitted development or apply for planning permission?
PD is faster (8 weeks for a Lawful Development Certificate vs 8–13 weeks for planning) but has hard size limits and excludes many conservation areas. Full planning unlocks larger and more contextual designs — usually worth the extra time on Victorian terraces in SW or W postcodes where you want a wraparound or contemporary glazing.
What does '2VP fixed price' actually mean?
Our quote becomes the contract sum on signing. Variations only ever go down unless you change the spec yourself (e.g. upgrade tiles, add joinery). No 'provisional sums' that quietly inflate. The contract programme is fixed too — and is backed by a 10-year insurance-backed HomePro guarantee.
Do you cover my borough?
Our core extension territory is Fulham (SW6), Chelsea (SW3), Notting Hill (W11), Chiswick (W4), Battersea (SW11), Hammersmith (W6), Clapham (SW4), Wandsworth (SW18), Wimbledon (SW19), Putney (SW15) and Paddington (W2). We work across Zones 1–3 and selectively beyond.
Extension cost in 59 London areas.
Twenty minutes with Nick Elias RIBA.
On us.
Bring your floorplan, sketches, or a single Pinterest pin. Walk out with a feasibility verdict, planning route, realistic budget and a programme — even if we’re not the right fit.
Studio at 20 Dawes Road SW6, video call, or on site.
One number. Sixty seconds. No callback.
Price your extension using the same 2026 rate card we quote real clients with.
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