How much does a single-storey extension cost?
Extensions are priced by floor area. Enter your extension size below for a region-adjusted all-in estimate.
What drives extension cost
- 1Size and complexity
Cost scales with floor area; complex rooflines and lots of glazing add per-m².
- 2Fit-out
A kitchen or bathroom in the extension is a big extra beyond the shell.
- 3Groundworks and services
Foundations, drainage diversions and structural openings into the existing house.
Extensions add floor area, which is the most direct lever on value — but they're also where budgets most often overrun. A provisional sum for groundworks and a proper contingency are essential in the schedule of works.
Single-storey extension costs (indicative)
| Size | Typical use | Indicative cost |
|---|---|---|
| 15 m² | Small kitchen/dining | £27,000 – £45,000 |
| 20 m² | Kitchen-diner | £36,000 – £60,000 |
| 30 m² | Large open-plan | £54,000 – £90,000 |
| Per m² (London/SE) | add ~20–30% | £2,200 – £3,600/m² |
Excludes VAT reliefs; kitchen fit-out, roof lanterns and bifolds add cost.
Frequently asked questions
How much does a single-storey extension cost?+
A typical 20 m² single-storey extension costs around £36,000–£60,000 all-in, or roughly £1,800–£3,000 per m² (more in London and the South East). Kitchen fit-out and glazing add cost.
Do I need planning permission?+
Many single-storey rear extensions fall under permitted development within size limits, but check with your local authority — Article 4 areas and conservation areas differ.
Related tools & guides
Want to know how these figures are calculated? See our cost methodology.
Cost figures shown are indicative estimates, not quotations. You are responsible for verifying all costs (obtain contractor quotes) and any figures submitted to a lender. ScopeWise is a documentation tool, not financial, tax, structural or planning advice. HMO compliance prompts are guidance only — confirm requirements with your local council, as standards and licensing vary by authority.