House Extension Cost UK — Complete 2026 Guide

Average Extension Cost UK 2026

House extensions in the UK cost £1,500–£3,000 per square metre for a standard build quality. A typical single-storey rear extension of 20 square metres costs £30,000–£60,000 fully finished. London costs are 30–50% higher.

Single Storey Rear Extension — £20,000–£60,000

The most common extension type. Extends the rear of the property on the ground floor. Typical sizes are 3–6 metres deep and the full width of the house. Under permitted development rules, most rear extensions up to 4m (detached) or 3m (semi/terrace) don't need planning permission.

Double Storey Extension — £40,000–£120,000

Extends both ground and first floor, creating two new rooms. More cost-effective per square metre than single storey. Almost always requires planning permission. Adds significant value but is more disruptive to build.

Side Return Extension — £25,000–£55,000

Fills in the narrow side alley common on Victorian terraced houses. Typically adds 8–15 square metres to the kitchen. Transforms a dark rear kitchen into a large, light space. Very popular in London and urban areas. Often permitted development.

Wrap-Around Extension — £50,000–£120,000

Combines a rear and side return extension into an L-shaped addition. Creates maximum ground floor space but is the most expensive single-storey option. Almost always requires planning permission due to the size.

What Affects the Cost

Key cost factors: floor area (the biggest variable), specification (budget, standard, or premium finishes), structural complexity, groundwork conditions, windows and glazing specification, and location. Always get at least 3 quotes from experienced builders.

Planning Permission

Extensions under permitted development don't need planning permission. Key limits: single storey rear extensions up to 4m (detached) or 3m (semi/terrace) from the original rear wall, maximum 4m height, must not exceed 50% of original garden area. Check permitted development rights on the Planning Portal.

Building Regulations

All extensions require building regulations approval for structural safety, insulation, electrics, and drainage. Your builder handles the application. Budget £600–£1,500 for building control fees.