House Extension Cost Guide UK 2026

A house extension adds ground-floor space (and sometimes a first floor above it). Cost depends on size, specification (basic vs high-end finishes), and structural complexity. Most single-storey rear extensions under 8 metres deep fall under Permitted Development and don't need planning permission.

Cost per square metre

Extensions are priced by floor area. Typical UK rates for 2026:

• Basic spec (£1,300-£1,500/sq m): standard bricks, UPVC windows, laminate worktops, budget tiles
• Mid-range (£1,800-£2,000/sq m): facing brick, aluminium bi-fold doors, quartz worktops, mid-range tiles
• High spec (£2,300-£2,700/sq m): engineered brick, large-format slim-frame glazing, natural stone, underfloor heating

A 20 sq m single-storey extension (typical for a kitchen extension) costs £28k-£50k depending on specification.

What affects the cost

Foundations

Foundations cost £100-£150/sq m. Clay soil needs deeper foundations (1.0-1.2m) than sand or gravel (0.8-1.0m). If the extension is near a tree, Building Control may require deeper foundations or root barriers, adding £2k-£5k.

Roof type

Flat roofs (fibreglass or EPDM rubber) cost £80-£120/sq m. Pitched roofs with tiles cost £120-£180/sq m. Roof lanterns (glass roof lights) add £1,500-£4,000 depending on size.

Windows and doors

A standard 3-panel bi-fold door (2.4m wide) costs £2,000-£3,500 for UPVC, £3,500-£6,000 for aluminium. Slim-frame glazing with minimal sightlines costs 30-50% more than standard aluminium. Floor-to-ceiling glazing across a full rear wall costs £8k-£15k.

Knocking through

Opening up the rear wall of your house (to connect the extension to your existing kitchen or dining room) requires a steel beam to support the floor and roof above. A 4-metre steel beam costs £800-£1,200 (steel + installation). A 6-metre beam costs £1,500-£2,500. Building Control checks the structural calculations.

Planning permission

Single-storey rear extensions up to 8 metres deep (detached houses) or 6 metres (terraced and semi-detached) fall under Permitted Development. The extension can't be higher than 4 metres, and it can't cover more than 50% of your garden. Side extensions must be single-storey and no wider than half the width of the original house.

If your extension goes beyond Permitted Development limits, you'll need planning permission. Application costs £258 (England, 2026). Allow 8 weeks for a decision.

Building Regulations

All extensions need Building Regulations approval. Building Control checks foundations, structural steelwork, insulation, ventilation, drainage, and electrical work. You can use your local authority Building Control (£800-£1,500 fee) or a private approved inspector (similar cost, sometimes faster).

Timeline

Design and planning: 4-8 weeks (longer if you need planning permission). Building work: 8-12 weeks for a single-storey extension, 12-16 weeks for double-storey. Weather delays the schedule (groundwork stops in heavy rain or frost).

Return on investment

A well-designed extension typically adds 10-20% to your property value, according to Nationwide. You'll usually recover the full cost when you sell. Extensions that create an open-plan kitchen/dining/living space are most desirable to buyers.

Sources: RICS Building Cost Information Service, Federation of Master Builders, Gov.uk Planning Portal, Nationwide House Price Index.