Single-storey extension cost UK 2026
A single-storey extension is the most common way UK homeowners add space, and the cheapest per project — you're building one floor, one roof and one set of foundations. This guide gives the 2026 cost per square metre by specification, the total for common sizes, and how rear, side and wrap-around layouts compare.
Cost per square metre
Specification drives the rate. The numbers below are build cost only, excluding VAT, and assume a UK-average location.
| Specification | £/m² | Typical for a 20 m² build |
|---|---|---|
| Basic — flat roof, simple plan, standard finishes | £1,700 – £2,000 | £34,000 – £40,000 |
| Standard — lantern or pitched roof, patio doors, mid-range kitchen | £1,900 – £2,600 | £38,000 – £52,000 |
| Premium — structural glazing, glazed roof, bespoke fit-out | £2,800 – £3,500 | £56,000 – £70,000 |
Total cost by size
Standard specification, UK average. The all-in column adds professional fees, 20% VAT on the build and a 12% contingency.
| Size | Rough dimensions | Build (ex-VAT) | All-in guide |
|---|---|---|---|
| 15 m² | 3m × 5m | £28,000 – £39,000 | £40,000 – £55,000 |
| 20 m² | 3m × 6.5m | £38,000 – £52,000 | £55,000 – £75,000 |
| 25 m² | 4m × 6.25m | £48,000 – £65,000 | £68,000 – £92,000 |
| 30 m² | 5m × 6m | £57,000 – £78,000 | £80,000 – £110,000 |
| 40 m² | 5m × 8m | £76,000 – £104,000 | £105,000 – £145,000 |
| 50 m² | 6m × 8.3m | £95,000 – £130,000 | £130,000 – £180,000 |
Rear vs side vs wrap-around
| Layout | Typical £/m² | Why |
|---|---|---|
| Rear extension | £1,900 – £2,600 | The baseline single-storey job |
| Side return | £1,900 – £2,200 | Small area, often infilling an existing return |
| Wrap-around (side + rear) | £2,400 – £3,000+ | Extra structural junctions and complex roof |
Regional adjustment
London +25–40%, South East +10–25%, South West +5–15%, Midlands around average, North England −5–10%, Wales −10–15%, Scotland around average with cities higher, Northern Ireland below average.
Fees, VAT and contingency
On top of the build: architect/designer £1,500–£3,000, structural engineer £400–£2,000, planning fee £258–£548, Building Regulations £400–£1,000, party wall surveyor £700–£2,000 where needed. Add 20% VAT to the build (extensions are standard-rated) and a 10–15% contingency. Fees and admin together usually total £3,000–£6,500.
Planning and timeline
Many single-storey rear extensions are Permitted Development: for a detached house, up to 4 m deep, or 8 m under the larger home extension scheme with prior approval, within height limits. Building Regulations approval is always needed. On site, expect 10–14 weeks; design and approvals beforehand add two to four months.
Frequently asked questions
How much does a single-storey extension cost in the UK in 2026?
About £1,700–£2,000/m² basic, £1,900–£2,600/m² standard, £2,800–£3,500/m² premium. A typical 20 m² rear extension is around £40,000–£60,000 for the build before VAT and fees.
How much is a 20m² single-storey extension?
Roughly £38,000–£52,000 for the build at standard spec, or £55,000–£75,000 all-in with fees, VAT and contingency. London and the South East add 15–40%.
What is the cost per square metre of a single-storey extension?
£1,900–£2,600/m² is the 2026 standard-spec benchmark, with a regional adjustment on top.
Do I need planning permission for a single-storey extension?
Often no — many fall under Permitted Development within depth and height limits. Building Regulations approval is still required.
How long does a single-storey extension take to build?
Around 10–14 weeks on site, plus two to four months for design and planning beforehand.
A quick note
Indicative 2026 UK figures from published cost guides. Your drawings, site and local market decide the real number — get three like-for-like quotes. See our full disclaimer.