House Extension Costs UK 2026

Cost per square metre

Extension costs are usually quoted per square metre of internal floor area. Double-storey extensions cost less per m² than single-storey because the expensive foundations and roof are shared across two floors. London and the South East sit at the top of the ranges.

• •Single-storey (basic spec): £1,800–£2,500 per m².
• •Single-storey (high spec / lots of glazing): £2,500–£3,500 per m².
• •Double-storey: £1,500–£2,500 per m².
• •Wrap-around (side + rear): priced as single + double combined.

What moves the price

Large sliding or bi-fold doors, rooflights, and high-end kitchens push a basic shell cost up sharply. Ground conditions matter too — poor ground or a sloping site can add £10,000+ to foundations. Don't forget to budget for VAT (most builders quote ex-VAT unless stated), planning and building control fees, and an architect or designer.

• •Glazing: bi-fold doors and rooflights add thousands.
• •Foundations: poor ground, trees or drainage can add cost.
• •Fees: architect, structural engineer, planning, building control.
• •VAT: confirm whether quotes include it.

Planning and permitted development

Many rear extensions fall under Permitted Development if they meet size limits (single-storey rear up to 4m on a detached house, 3m on others, or 8m / 6m under the larger home scheme subject to neighbour consultation). Anything beyond that, or in a Conservation Area, will need full planning permission. See our planning guide for details.