Typical costs in 2026
| Extension type | Typical cost range |
|---|---|
| Single-storey (basic to mid-spec) | £25,000–£60,000 |
| Double-storey | £45,000–£120,000 |
| Cost per m² (basic) | £1,800–£2,200/m² |
| Cost per m² (mid-spec) | £2,200–£2,800/m² |
| Cost per m² (high specification) | £2,800–£3,500/m² |
These figures reflect Greater London and the South East. Expect 10–15% lower outside London.
What affects the cost
Size is the primary driver — but specification matters as much. Key factors: structural complexity (steels, underpinning), M&E (full rewire, new heating circuits), quality of finishes (bifold doors, roof lights, underfloor heating), planning fees and surveys, and site access.
Planning permission
Most single-storey rear extensions are permitted development — no planning application required — if they stay within 4m (detached house) or 3m (semi-detached or terraced) from the original rear wall. Extensions beyond this need planning consent (typically 8 weeks).
Getting accurate quotes
Get at least 3 written quotes based on the same detailed scope of works. A quote without a site visit is unreliable.