Extension cost calculator · London · May 2026
How much does a house extension cost in London?
A house extension in London costs £2,500–£5,000 per square metre for the build (May 2026), or £75,000–£250,000 all-in for a typical single-storey extension once design fees, VAT and a contingency are included. Side returns sit at the top of the per-metre range and rear extensions at the bottom. A loft adds a bedroom for £60,000–£150,000. Build Team has run 1,750+ London projects since 2007. The figures here come from our own completed projects and online quotes, cross-checked against the RICS Building Cost Information Service (BCIS), the HomeOwners Alliance and the Federation of Master Builders. The calculator below uses the same size-band rates we quote from, and shows your estimate before it asks for an email.
Last updated: 15 September 2026 · Prices reviewed quarterly against the BCIS index
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Build cost, inc. VAT, excluding design fees, party wall and contingency.
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Extension costs in London by type (May 2026)
| Extension type | Build cost per m² | Typical all-in | Typical size |
|---|---|---|---|
| Side return extension | £3,500–£5,000 | £80,000–£140,000 | 10–18 m² |
| Rear extension | £2,500–£4,200 | £75,000–£150,000 | 15–25 m² |
| Wraparound extension | £3,000–£4,800 | £130,000–£250,000 | 20–35 m² |
| Kitchen extension | £2,500–£4,200 + kitchen | £90,000–£180,000 | 15–25 m² |
| Loft conversion | £2,500–£5,500 | £60,000–£150,000 | 20–30 m² |
| Loft extension (roof raised) | £3,000–£4,500 | £120,000–£220,000 | 30–45 m² |
Build cost is construction only. All-in includes design and planning fees, structural engineer, Building Control, party wall surveyor, VAT at 20% and a 10–15% contingency. Figures correct as of May 2026, cross-checked against BCIS, Architecture for London and Homebuilding & Renovating 2026 cost data and our own completed projects.
What moves the price
Size
The single biggest factor. Cost per square metre falls as the extension grows, because foundations, steel and roof are shared over more floor area. Our size bands reflect that.
Specification
Rooflights versus a structural-glass lantern, UPVC versus slim aluminium doors, an off-the-shelf kitchen versus made-to-measure. Between a mid-range and a high-end finish there can be £2,000/m² of difference.
Type of extension
A side return costs more per metre than a rear extension: it is built from inside a narrow plot, almost always needs a party wall award, and the steel spans are longer.
Planning route
Permitted development is cheaper and faster than a full application. Conservation areas and Article 4 areas (parts of Hackney, Camden, Islington, Lambeth, Westminster) add design constraints and 4–8 weeks.
Ground and drainage
Shallow drains, clay soil or a tree near the boundary mean deeper foundations. It is the one genuine unknown, which is why we carry a contingency rather than a surprise.
Why national guides look cheaper
Checkatrade, the FMB and the HomeOwners Alliance quote £1,800–£3,000/m² as a UK rule of thumb. Those are national averages for the shell. London labour is 25–40% higher and sites are tighter, so any London figure under £2,000/m² is either out of date or shell-only.
Borough
Labour and access cost more in inner boroughs. A rear extension in Wandsworth and the same one in Islington can differ by 10–15% before the planning route is considered.
Real Build Team projects and what they cost
| Project | Where | What we built | Size | Build cost | Per m² |
|---|---|---|---|---|---|
| Milton Road SE24 | Lambeth (conservation area) | Ground-floor side + first-floor rear | 20 m² | £74,000 + VAT | £3,700/m² |
| Bonham Road SW2 | Lambeth | Side return, high finish | 52 m² | £109,000 + VAT | £2,100/m² |
| Cricklade Avenue SW2 | Lambeth (Leigham Court conservation area) | Side return extension | 43 m² | £91,000 + VAT | £2,100/m² |
| Mayfield Road N8 | Haringey | Wraparound extension | 50 m² | £87,000 + VAT | £1,750/m² |
| Nightingale Mansions SW12 | Wandsworth | Rear extension | 66 m² | £85,000 + VAT | £1,300/m² |
| Antill Road N15 | Haringey | Side infill extension | 26 m² | £69,000 + VAT | £2,650/m² |
| Scholars Road SW12 | Wandsworth | Ground floor extension, shell and core | 36 m² | £70,000 + VAT | £1,950/m² |
| Heber Road SE22 | Southwark | Wraparound extension | 45 m² | £61,600 + VAT | £1,370/m² |
| Lidfield Road N16 | Hackney | Rear extension | 35 m² | £59,000 + VAT | £1,690/m² |
| Therapia Road SE22 | Southwark | Rear extension, 6-bed detached, panorama doors | — | £150,000 + VAT | — |
From our published case studies, built between 2019 and 2025; earlier projects reflect the prices of their year, which is why some per-metre figures sit below today's £2,500–£5,000 range. Build cost excludes VAT and professional fees. Read the full case studies or browse the project gallery.
What London homeowners are actually quoted
Aggregate figures from 12,394 ground-floor extension estimates generated by this calculator, October 2026. Median build cost, inc. VAT, by postcode area.
Medians of estimates requested by homeowners, not final contract values; they include the size and specification each person chose. Updated daily.
What the estimate includes, and what it doesn't
Included
- Structure, foundations, steel and brickwork
- Roof, rooflights or lantern as chosen
- Doors and glazing as chosen
- Electrics, plumbing and heating first and second fix
- Plastering and decoration of the new space
- VAT at 20%
Not included
- Design and planning fees (Design Phase from £2,995 + VAT)
- Party wall surveyor, £800–£2,000 per neighbour
- Kitchen units and appliances above a mid-range allowance
- Landscaping and garden reinstatement
- Contingency (we recommend 10–15%)
- Work to the rest of the house
Frequently asked questions
For the build alone, £2,500–£5,000 per square metre depending on type and specification: rear extensions from about £2,500/m², side returns £3,500–£5,000/m² because of the party wall and structural work on a narrow plot, lofts £2,500–£5,500/m². Add professional fees, VAT at 20% and a contingency for the all-in figure.
Design and planning fees, structural engineer, Building Control, party wall surveyor, the build, VAT at 20% and a 10–15% contingency. It does not include kitchen units above a mid-range allowance, landscaping, or work to the rest of the house.
It uses the same size-band rates we quote from, so for a standard site it lands within about 15% of a written quote. What moves it: ground conditions, drainage, a structural-glass roof, and the kitchen you choose. The fixed price follows a free consultation and a site survey.
Labour rates are 25–40% higher, sites are tight (a side return on a terrace is built from inside the plot), and party wall work is almost always needed. Planning is also more constrained in conservation areas and Article 4 areas.
Per square metre, often yes: a rear dormer at £75,000–£95,000 all-in gives you a bedroom and bathroom without touching the garden. A ground-floor extension costs more per square metre but adds the room most families want, the kitchen-diner.
Yes, the total shown includes VAT at 20%. Most domestic extensions do not qualify for reduced-rate VAT.
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