House ExtensionCost Calculator

A single-storey rear extension in the UK costs £1,800–£2,800 per m² in 2026, meaning a typical 20 m² extension costs £36,000–£56,000. Double-storey extensions cost £1,600–£2,500/m² — cheaper per m² because the roof and foundations are shared across two floors. Wrap-around extensions (rear + side return) cost £2,200–£3,500/m² due to their complexity. High-specification extensions with structural glazing cost 50–60% more than standard spec.

Most single-storey rear extensions fall under Permitted Development (PD) rights and do not require planning permission, provided they meet size limits: max 3m projection for semi-detached, 4m for detached houses (or 6m/8m under the Neighbour Consultation Scheme). Side extensions, extensions in conservation areas, listed buildings, or those exceeding PD limits require full planning permission, which typically costs £206 (England) and takes 8–13 weeks to determine. Always check with your local planning authority first.

A standard single-storey rear extension takes 10–16 weeks to build from groundworks to completion. Groundworks and foundations take 2–3 weeks. Brickwork and structural frame take 3–4 weeks. Roof and weatherproofing take 2–3 weeks. Internal first fix (plumbing, electrics) takes 2 weeks. Plastering, second fix, and decoration take 3–4 weeks. Factor in 2–4 weeks' lead time for bi-fold or sliding doors if ordering custom sizes.

The cost per m² for a house extension includes: groundworks and foundations, structural walls (brick, block or timber frame), roof structure and covering, windows and external doors, first-fix plumbing and electrics, plastering, screed floor, and basic decoration. It excludes: planning fees, architect or structural engineer fees (typically £2,000–£8,000), internal fit-out (kitchen, bathroom), flooring, furniture, and landscaping to the rear garden after construction.

Yes — a structural engineer is required for any extension that involves removing or modifying a load-bearing wall (to open the extension into the existing house), deep or complex foundations, or any steel beams. Structural engineer fees typically cost £500–£2,000 for calculations and drawings. Building Regulations approval is mandatory for all extensions regardless of planning permission status — your builder will liaise with your local authority Building Control or an Approved Inspector.

Double-storey extensions offer the best value per m² of added floor space — at £1,600–£2,500/m² compared to £1,800–£2,800/m² for single-storey — because the foundations, roof, and scaffolding costs are shared across two floors. A 20 m² footprint double-storey adds 40 m² of space for perhaps 30–40% more than a single-storey option. Single-storey rear extensions are the most straightforward and have the fewest planning complications. Side-return extensions are popular in Victorian terraces where they can transform a small galley kitchen into a large open-plan space.