House Extension Cost Calculator UK

A single-storey rear extension typically costs £1,500-£2,500 per m² depending on finish quality. Two-storey extensions are cheaper per floor at £1,200-£2,000/m² because you share the foundation and roof costs across two floors. Side extensions run £1,400-£2,200/m², wrap-arounds £1,600-£2,800/m², and over-garage conversions £1,000-£1,800/m². London adds 25-30% to these figures.

Many single-storey rear extensions fall under Permitted Development rights and do not need planning permission. The rules allow up to 3m for attached houses or 4m for detached (doubled under Prior Approval). However, you must not exceed 4m in height, cover more than 50% of the garden, or extend beyond the front elevation. Conservation areas and listed buildings have stricter rules. Always check with your local council.

A single-storey rear extension typically takes 10-14 weeks from groundworks to completion. Two-storey extensions take 14-20 weeks. Wrap-around extensions take 14-18 weeks. These timelines assume good weather and no planning delays. Add 8-12 weeks for planning permission if required, plus 4-6 weeks for Building Regulations approval. Party wall agreements can add 2-3 months.

Open-plan kitchen-diners, en-suite bedrooms, and extra bathrooms add the most value. A well-designed extension can add 10-20% to your property value. Bifold doors to the garden, underfloor heating, and high ceilings are popular features. The key is ensuring the extension complements the existing house and is built to a good standard.

The cost per m² typically includes foundations, walls, roof, windows, doors, basic electrics, plumbing, plastering, decoration, and project management. It usually excludes: kitchen fitting, bathroom fitting, structural steel, demolition of existing structures, and any specialist finishes. These extras are quoted separately and can add 30-50% to the base build cost.

Yes, almost all extensions require Building Regulations approval regardless of whether planning permission is needed. Building Regs cover structural integrity, fire safety, thermal performance, ventilation, drainage, and electrical safety. You can use a Local Authority Building Control officer or an approved private inspector. Failing to get Building Regs signed off causes problems when selling.

A kitchen within an extension adds £8,000-£20,000 depending on size and specification. A basic galley kitchen with standard units costs £8,000-£12,000. A mid-range kitchen with stone worktops and integrated appliances costs £12,000-£16,000. A premium kitchen with bespoke cabinetry and high-end appliances costs £16,000-£20,000+. These figures include fitting.

Wrap-around extensions (combining side and rear) are the most expensive per m² but create the most dramatic transformation. They work particularly well on semi-detached and detached houses where you can extend both sideways and backwards. The extra cost compared to a simple rear extension is usually 20-40%, but you get significantly more space and often a better layout.

Structural steel is commonly needed when removing walls between the existing house and the extension, or when creating wide openings for bifold doors. A structural engineer designs the steel work (£500-£1,000 for calculations), and the steel itself plus installation costs £2,000-£5,000. Underpinning or piling may be needed on poor ground or near trees, adding £5,000-£15,000.

Location significantly affects costs through labour rates and land values. London and the South East add 15-30% to national average costs. The Midlands and South West are close to average. The North, Wales and Scotland are typically 10-15% below average. Within cities, access difficulty (narrow streets, limited parking for skips) can also increase costs.