Extension cost per m² in London (2026 ranges, by extension type)
Working square-metre rates for side returns, rear, two-storey and wrap-around extensions in Greater London, with the spec variables that move the number.
Key facts
- Side return: £3,200–£4,000/m² (Greater London, mid spec).
- Single-storey rear: £3,000–£4,200/m².
- Two-storey rear: £3,400–£4,800/m².
- Wrap-around (side + rear): £3,600–£4,800/m².
- Prime central postcodes: add 15–20% to all rates.
- Glazing spec swing: £400–£800/m² between standard aluminium and Crittall / structural glass.
Per-square-metre rates are a starting point, not a quote. They give you the order of magnitude in five seconds, against which you can test whether a contractor's number is sensible. The numbers below are the working ranges we see across our London portfolio in 2026.
Glazing spec, finish spec and party-wall complexity move the number more than the extension footprint does.
Rates by extension type
| Extension type | Mid spec | Prime spec | Prime central | |---|---|---|---| | Side return | £3,200–£3,600/m² | £3,800–£4,500/m² | +15–20% | | Single-storey rear | £3,000–£3,600/m² | £3,800–£4,400/m² | +15–20% | | Two-storey rear | £3,400–£4,000/m² | £4,200–£4,800/m² | +15–20% | | Wrap-around | £3,600–£4,200/m² | £4,400–£5,200/m² | +15–20% | | Basement extension | £4,500–£6,500/m² | £6,500–£9,000/m² | +20–25% |
Mid spec = good kitchen, decent glazing, underfloor heating to ground floor, mid-range sanitaryware.
Prime spec = bespoke kitchen, structural or Crittall glazing, designer specification throughout, full M&E refresh.
What moves the number most
In order of impact:
- Glazing. Standard aluminium sliding doors vs Crittall vs frameless structural glass, £400–£800/m² swing on the whole extension.
- Kitchen and bathroom spec. A £25,000 kitchen vs a £80,000 kitchen on a 35 m² extension is £1,500/m².
- Party wall and structural complexity. A side return between two existing return walls is straightforward. A side return involving underpinning to a neighbour's basement is not.
- Floor finish. Engineered oak at £80/m² vs limestone at £250/m² is a £6,000 swing on a 35 m² floor area.
- Bifold/sliding door spec. A £6,000 set vs a £18,000 set is real money on a small footprint.
What the rates include and exclude
Included: contractor's labour and materials, preliminaries, contractor OHP, scaffolding, skip and waste, sub-contract trades, snagging.
Excluded: VAT (20% on most domestic work), architect and engineer fees, party wall surveyor fees, planning fees, building control, cost expert fees, FF&E, kitchen appliances supplied by the homeowner.
Using the rates against a contractor quote
If a contractor returns £4,800/m² for a standard single-storey rear extension in zone 3 London with mid spec, the number is above the range. Either the spec is being upgraded, the site is unusually difficult, or the contractor is loading the price. The fastest way to find out is to issue the same drawings to a second contractor, or to run an independent cost check before signing.
Frequently asked
- What is included in the £/m² figure?
- Construction cost only: contractor's price including labour, materials, prelims and OHP. Excluded: VAT, professional fees (architect, structural engineer, cost expert, planning), party wall, building control, FF&E, kitchen appliances supplied by homeowner.
- What size extension makes financial sense?
- Below 15 m², the fixed costs (prelims, scaffolding, party wall) dominate and the £/m² rate balloons. Sweet spot is 25–60 m² for most London terraced houses.
- How much contingency should I hold for a London extension?
- 10–15% on a refurbishment-style extension where existing walls are opened up. 7–10% on a clean new-build extension with no fabric risk.
- Will I need a structural engineer and a cost expert?
- Structural engineer: yes, for almost any extension involving a new opening or steel beam. cost expert: strongly recommended above £150,000, and for any project where you are quoteing to more than one contractor.
- Are these rates higher than two years ago?
- Yes, by roughly 8–12% versus 2024. Labour has driven most of the increase; materials have stabilised.
