House extension cost in London
Pick the shape, the size and the finish. The calculator gives a London guide price for a modular extension, then adds the crane, the professionals and the fees most London jobs need.
- London rate per m², incl. VAT
- £2,500 to £4,600
- National rate per m²
- £2,100 to £3,300
- Party wall surveyor, per neighbour
- £700 to £1,500
A guide, not a quote.
London guide price, including VAT
£59,500 to £82,500
Rear extension, 18 m², mid-range finish
A guide, not a quote. Typical national ranges for 2026 including VAT, with the usual London allowance of 20 to 40% added. Kitchen, bathroom and furniture not included.
London guide prices by job
Build cost only, across all three finishes, for the extensions London terraces most often get.
| Extension | Floor area | London guide, incl. VAT |
|---|---|---|
| Small side return infillNarrow strip beside the back addition | 8 m² | £20,000 to £37,000 |
| Full side returnAlong the whole back addition | 12 m² | £30,000 to £55,500 |
| Rear extension, terrace widthAbout 3 m deep across a 5 to 6 m plot | 18 m² | £45,500 to £83,000 |
| WraparoundSide return and rear in one L | 28 m² | £70,500 to £129,500 |
A guide, not a quote. Typical national ranges for 2026 including VAT, with the usual London allowance of 20 to 40% added.
London rates by finish
These are the national 2026 rates for a finished modular extension, with the 20 to 40% London allowance applied. Groundworks and the standard foundation are inside the rate; the crane is not.
| Finish | What you get | London per m² |
|---|---|---|
| Plain and practical | Simple aluminium doors, one roof light, standard floor and lighting | £2,500 to £3,500 |
| Mid-range | Wide sliding doors, a run of roof glazing, better floor and joinery | £3,000 to £4,050 |
| High specification | Slim-frame glazing, bespoke cladding and joinery | £3,500 to £4,600 |
Why the spread is wide: the low end is outer-borough work with easy access; the top is a central street with a road closure, a tight crane swing and a high finish.
Fees on top of the build
Professional fees usually run to 10 to 15% of the build cost, and a contingency of another 10 to 15% is sensible on any older London house. The ones you are almost certain to meet:
- Planning drawings: £1,000 to £2,500 for a single-storey extension.
- Structural engineer: £600 to £1,200 for drawings and calculations.
- Building control: about £700 to £1,100 including VAT. Each borough sets its own charges.
- Party wall surveyor: £700 to £1,500 per neighbour with one agreed surveyor, nearer £1,500 to £3,000 where each side appoints its own. You pay for both.
- Planning: £548 for a householder application, £274 for a lawful development certificate, or £249 for prior approval of a larger rear extension.
What moves a London price
The crane and the street
A one-day contract lift with a 25 tonne crane, appointed person, supervisor and slinger comes to about £1,400 as a national guide. City and compact cranes for tight streets run about £600 to £1,200 a day bare. On top come the borough’s parking suspension and, on some roads, a road closure. Those charges vary by borough, so ask the builder to include them in the price.
Opening up the back of the house
Most side returns and rear extensions remove part of the back wall. A steel beam supplied and fitted is typically £1,200 to £2,500, more where two beams meet at a corner in a wraparound.
The ground
London clay shrinks and swells, and street trees and big garden planes make it worse. Where the engineer finds that, deeper footings or piles may be needed. Piles are designed after a ground investigation and can add several thousand pounds.
Kitchen and bathroom
A kitchen fitted inside the extension adds roughly £15,000 to £35,000; a bathroom £5,500 to £9,000.
Sources and how we use them
A guide, not a quote. Typical national ranges for 2026 including VAT, with the usual London allowance of 20 to 40% added. Build rates and professional fees are national ranges from 2026 cost guides; planning fees are set by MHCLG from 1 April 2026; building control fees by each council.
London cost questions
How much does a house extension cost in London?
For a finished single-storey modular extension, allow £2,500 to £4,600 per m² including VAT. That is the national 2026 range of £2,100 to £3,300 with a London allowance of 20 to 40%. A typical 18 m² rear extension on a terrace lands somewhere between £45,500 to £83,000 before fees.
Why does London cost more?
Labour, parking, waste and access all cost more, and terraced plots add party wall work and tighter logistics. The factory part of a modular build costs much the same wherever the house is; the site work is where London adds its premium.
Does the calculator include a kitchen?
No. A new kitchen inside the extension usually adds £15,000 to £35,000, and a bathroom £5,500 to £9,000. Both are national ranges.
What could push the price higher?
Poor ground or trees close to the house (piles may be needed, designed by an engineer after a ground investigation, and they can add several thousand pounds), a steel beam to open up the back of the house, a conservation area that asks for brick or timber to match, and a second crane day.
Get a quote for your extension
Postcode, four quick questions, then your contact details. It takes about a minute.
We are an introduction service, not a builder. We look for a firm that works in your borough, tell you who it is, and pass your details on only when you say yes.
Got it. Thank you.
- We look for an extension builder who takes on jobs like yours in your part of London, and we email or phone you with the firm's name.
- If you are happy with that firm, we send it what you told us. If not, nothing goes anywhere.
- The firm arranges a visit, looks at the access and the ground, and gives you its price.