How Much Does a Rear Extension Cost in 2026?
The average rear extension in the UK costs between £45,000 and £90,000 in 2026, working out to roughly £1,800–£2,500 per m². Get 3 free quotes from vetted local builders and compare real prices for your project.
Rear extension cost at a glance
The average cost of a rear extension in the UK in 2026 is £45,000–£90,000, depending on size, finish and location.
- Small rear extension (15m²): £27,000 – £40,000
- Medium rear extension (25m²): £45,000 – £65,000
- Large rear extension (40m²): £72,000 – £100,000
- Per m² average: £1,800 – £2,500
Prices include materials, labour, building control and VAT, but exclude kitchen fit-out and planning fees. London and the South East typically sit 15–25% above the UK average.
2026 Rear Extension Cost Breakdown
Rear extension prices vary based on size, specification and where you live. Use the table below as a realistic 2026 starting point, then get 3 free quotes for prices specific to your project.
| Extension Size | Typical Use | 2026 Cost Range |
|---|---|---|
| Small (10–15m²) | Kitchen extension, utility area | £27,000 – £40,000 |
| Medium (20–25m²) | Open-plan kitchen-diner | £45,000 – £65,000 |
| Large (30–40m²) | Kitchen, dining & living combined | £72,000 – £100,000 |
| Wrap-around (40m²+) | Rear + side extension combined | £85,000 – £140,000 |
| Double-storey rear | Extra bedroom + downstairs space | £90,000 – £160,000 |
Prices include foundations, structural work, roofing, windows, doors, plastering, electrics, plumbing, building control and VAT. Kitchen or bathroom fit-out is priced separately.
6 Factors That Change Your Rear Extension Cost in 2026
London and the South East are typically 15–25% above the UK average. Northern England, Wales and Scotland often come in 10–15% lower. Get quotes from local vetted builders to see real regional prices.
Bifold or sliding doors, lantern roofs, flush floors and underfloor heating all add significant cost. A high-spec 25m² extension can exceed £80,000; a standard finish is around £55,000.
Deep foundations, diverted drains, tree root protection or proximity to neighbours can add £3,000–£8,000. A structural engineer's report will flag most issues early.
Knocking through to create open-plan space typically needs RSJs at £1,500–£4,000 per opening, plus Building Control sign-off. Larger openings can double the cost.
Permitted Development extensions are cheaper and faster. Full planning applications cost £258 (householder) plus architect and drawings — often £2,500–£4,500 total for full plans.
Brick, timber and steel have stabilised in early 2026 after the 2023–24 spikes. Glazing has risen ~3%. Labour rates are up 4–6% year-on-year. Budget with a 10% contingency.
More 2026 Extension & Building Cost Guides
All cost guides →Full cost guide for kitchen extensions including fit-out, appliances and glazing.
Velux, dormer, hip-to-gable and mansard — 2026 price breakdown with ROI data.
Single-storey, double-storey, wrap-around — every extension type compared.
When you need planning permission and when you can use Permitted Development.