"How much per square metre?" is the first question we get asked at every site visit — and the honest answer is: it depends on structure, glazing and finishes far more than on postcode. That said, after a decade of building extensions across East London and Essex, prices in 2026 cluster into predictable ranges.
Typical London extension costs in 2026
The figures below are build costs for a decent mid-range specification, excluding VAT, professional fees and fittings like kitchens:
| Extension type | Typical 2026 range |
|---|---|
| Single-storey rear extension | £2,200 – £3,000 per m² |
| Side-return extension (terraced) | £2,400 – £3,200 per m² |
| Wrap-around extension | £2,300 – £3,000 per m² |
| Double-storey extension | £1,900 – £2,600 per m² (per floor) |
| Loft dormer conversion | £1,800 – £2,500 per m² |
Indicative ranges for budgeting only — every site is different. A fixed, itemised quotation is the only number worth planning around.
What actually drives the price
- Structural openings. Every steel beam and the temporary works behind it adds cost — a fully open-plan rear with a hidden steel costs more than two supported openings.
- Glazing. Sliding and bifold doors, rooflights and slim-frame systems are often the single biggest line after the shell.
- Kitchens and bathrooms. The same extension can carry a £12k kitchen or a £45k one.
- Groundworks and drainage. Poor ground, nearby trees, build-over agreements and moved drainage all add below-slab cost you never see.
- Access and logistics. Terraced streets with no side access mean hand-digging, grab lorries and slower programmes.
The costs people forget
- Planning or lawful development certificate fees, plus building control charges
- Structural engineer's design and party wall surveyor(s) where notices are needed
- VAT — most quotes you compare will exclude it
- A contingency of around 10% for the unknowns that only appear once things are opened up
Do you need planning permission?
Many single-storey rear extensions fall under permitted development, subject to depth, height and boundary limits — but flats, conservation areas and already-extended houses are common exceptions. A lawful development certificate is cheap insurance even when permission is not required.
We work as one team with your architect, structural engineer and the approving authority of your choice — LABC, NHBC or independent inspectors — so approvals do not stall the programme.
How to keep your budget on track
- Fix the scope in writing before work starts. Most overruns are scope creep, not builder error.
- Insist on a fixed, itemised quote — so any change is priced against a visible baseline.
- Choose finishes early. Late kitchen, glazing and tile decisions cause programme gaps that cost money.
- Use one accountable contractor from groundworks to decoration, so there is no gap between trades to fall into.
Written by Scope Design Construction LTD — family-owned design & build contractors in Wanstead, East London, delivering extensions, new builds and refurbishments since 2014.
