Calculate 2026 UK single-storey rear, side-return, or wrap-around house extension costs per square metre (m²) by width, depth, spec level, bi-fold doors, kitchen fit-out, and UK region.
In the UK in 2026, a single-storey house extension costs £1,900 to £3,100 per square metre (m²) for construction, plus 10%–15% for professional fees and VAT. A popular 4m × 5m (20 m²) rear kitchen-diner extension costs £42,000 to £62,000 outside London, or £54,000 to £78,000 in London and the South East.
📐 Engineering Formula Used: Total Cost (£) = (Width m × Depth m × Spec Rate × UK Region) + Bi-Fold/Lantern + Kitchen Fit-Out + RSJ Knock-Through
The table below provides verified 2026 material, labor, and installed turnkey cost ranges for single storey extension cost calculator UK projects. Compare these figures alongside live industry benchmarks such as RenoCost.co.uk & PriceNailer.com ↗.
| Extension Size (Metres) | Floor Area (m²) | Standard Spec (2026 UK) | Mid-Range Spec (2026 UK) | London High-Spec Total |
|---|---|---|---|---|
| 3m × 4m Small Rear Extension | 12 m² | £24,000 – £30,000 | £30,000 – £37,000 | £38,000 – £48,000 |
| 4m × 5m Kitchen-Diner Extension | 20 m² | £39,000 – £48,000 | £48,000 – £59,000 | £62,000 – £78,000 |
| 5m × 6m Large Family Extension | 30 m² | £57,000 – £70,000 | £70,000 – £86,000 | £90,000 – £115,000 |
| Side-Return Wrap-Around Extension | 35 m² | £68,000 – £84,000 | £84,000 – £102,000 | £108,000 – £135,000 |
Project Scenario: 5m Wide × 4m Deep (20 m²) Mid-Range Single-Storey Rear Extension with Bi-Fold Doors & Rear Wall Knock-Through
| Itemized Quote Line Item | 2026 Estimated Cost |
|---|---|
| 20 m² Extension Build — Trench Foundations, Cavity Walls, Roof & Plastering (@ £2,350/m²) | £47,000 |
| Rear Load-Bearing Wall Knock-Through & Structural Steel RSJ Box Frame | £4,200 |
| 3.6m Aluminium Bi-Fold Patio Doors & Double-Glazed Roof Lantern | £5,400 |
| Estimated Total UK Single-Storey Extension Cost | £56,600 (£2,830/m²) |
Under standard UK Permitted Development rights, you can build a single-storey rear extension up to 3 metres deep on an attached/semi-detached house or 4 metres deep on a detached house without full planning permission. Under the Prior Approval Larger Home Extension scheme, this doubles to 6 metres (attached) and 8 metres (detached) if neighbours do not object.
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