House extension cost in London (2026)
By Kristian Toth, Co-owner, Making World Beautiful · Last updated: 16 October 2026
Short answer
Extensions with MWB cost £2,000–£4,000 per m². A 20 m² rear extension is roughly £40,000–£80,000, usually 12–16 weeks on site.
| Extension | Typical cost | Time on site |
|---|---|---|
| Small rear (15 m²) | £30,000–£60,000 | 10–12 weeks |
| Side-return (20 m²) | £40,000–£80,000 | 12–14 weeks |
| Wraparound (30 m²) | £60,000–£120,000 | 14–18 weeks |
Indicative MWB prices including labour and materials. Your fixed price follows a free site visit.
Do I need planning permission for a rear extension in London?
Many single-storey rear extensions fit permitted development, but conservation areas and Article 4 directions often remove it. Side-returns and wraparounds usually need planning.
What hidden costs should I budget for?
Allow roughly 10–15% on top of the build price for fees and contingency. The most common extras in London are:
- Party wall surveyor: typically £700–£1,500 per neighbour if works affect a shared wall
- Structural engineer calculations: usually £500–£2,000
- Building control fees: commonly £800–£2,000 depending on the council
- Skip permits and parking suspensions: charged per week by the borough
- Planning or lawful development certificate fees where needed
Frequently asked questions
How much does an extension cost per m² in London?
With MWB, £2,000–£4,000 per m².
Will I need a party wall agreement?
Usually yes, if you build on or near a shared boundary. Budget £700–£1,500 per neighbour.