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Single-storey extension cost: what it really costs in 2026
Priced per square metre, honestly.
£2,400 – £4,800 /m²
Budget from about £2,400 per square metre for a standard finish, £3,600 premium, and £4,800 at architect-designed spec. A typical 15m² extension starts around £32,000 all-in.
Price by spec
| Option | What it means | Typical price |
|---|---|---|
| Standard spec | Good quality, straightforward design | £2,400 /m² |
| Premium spec | Higher-end glazing and finishes | £3,600 /m² |
| Architect-designed | Structural glazing, bespoke details | £4,800 /m² |
What actually happens
- Design, drawings and structural calculations
- Foundations dug and poured (the messy fortnight)
- Walls up, steels in where the house is opened
- Roof on and made watertight — the turning point
- Windows and doors fitted; first-fix services
- Plastering, screed and underfloor heating if chosen
- Second fix, decoration and the knock-through tidy-up
Popular choices
Bifold or sliding doorsRoof lanternsCrittall-style glazingPolished concrete or large-format tiles
Regulations to know
- Many rear extensions fall under permitted development (up to 4m, or 6m with neighbour consultation on semis/terraces) — beyond that, planning permission
- Building regulations always apply, with staged inspections
- Party Wall Act notices are usually needed when building near a boundary
Tips from the trade
- The watertight stage is the milestone that de-risks everything after it
- Hold 10–15% contingency; ground conditions are the classic surprise
Plan this project properly
Get a region-adjusted estimate, track budget against real quotes, and find local tradespeople -- free to browse on Qome.
Start planning on QomePrices on this page are approximate, region-adjusted guide figures built from cross-checked UK cost data. They exist to inform you, not to bind anyone. Actual prices are whatever you and your tradesperson agree, and a difference between a Qome guide figure and a real quote is not a fault in either.