What does a house extension cost in the UK?
The cost of a house extension in the UK varies enormously based on size, specification, location, and market conditions. As a broad guide for 2024, a single-storey rear extension with standard specification costs £1,800–£2,500 per m² in most of England and Wales. Double-storey extensions are marginally cheaper per m² (the foundations and roof are shared) at £1,600–£2,200 per m². London adds roughly 20–30% to these figures. A 20m² kitchen extension at standard spec might cost £36,000–£50,000 including VAT — though costs at both ends of the market exist depending on the builder and finishes chosen.
What affects the cost per square metre?
Several factors push costs up or down significantly. Soil conditions matter: clay soil in areas like London can require deeper, more expensive foundations. Structural complexity — removing load-bearing walls, installing steel beams — adds £2,000–£8,000. Glazing specification is a major variable: standard French doors cost £1,500–£2,000; a full-width aluminium bifold system can run £5,000–£12,000 for the same opening. Roof type (flat vs pitched) and covering (felt, rubber, GRP, sedum) affect both cost and longevity. The level of finish specified for kitchen, flooring, and lighting can easily double the cost of the interior.
Calculating the floor area
Before getting quotes, measure the footprint accurately. Use our area calculator to find the m² of the extension's floor plan. For a rectangular extension it is simply length × width. For an irregular shape, break it into rectangles, calculate each, and add them. Add the area of any internal walls you are removing to open up the existing house — this work is often quoted separately but should be in your budget. Remember that planning permission is typically required for extensions over certain sizes (currently 4m for detached, 3m for semi/terraced on a single storey rear extension, under Permitted Development rights — check your local authority).
Hidden costs most budgets miss
The build cost is just the start. Planning application fees (£206 for householder applications in 2024), architect and structural engineer fees (8–12% of build cost), building regulations fees (£600–£1,200 depending on size), and party wall agreements with neighbours (solicitor costs if disputes arise) all add up. Budget separately for: kitchen fit-out or bathroom suite if the extension houses one, new furniture, internal decorating and repainting of the existing house affected, and landscaping to reinstate the garden disturbed during the build. A 10–15% contingency on the build cost is not pessimistic — it is prudent.
How to get reliable quotes
Get a minimum of three written quotes from builders and compare them line by line, not just on total price. Check the scope of each quote matches: is VAT included? Does it cover disposal of excavated material? Does it include plastering and decorating or just the shell? Ask for a payment schedule tied to milestones, not arbitrary dates. Verify the builder is registered with a trade body (FMB, NHBC, or similar), carries public liability insurance, and can provide references from recent comparable projects. Avoid builders who want large upfront payments before work begins — the standard is a deposit of 10–20% maximum.
Using the project cost calculator
Our project cost calculator gives you a quick range estimate based on floor area, project type, and specification level. Use it to sense-check builder quotes — if a quote comes in significantly below the low end of the range, understand why before proceeding (it may mean missing scope). If quotes come in significantly above the high end, check whether you are in a high-demand area, whether the specification is higher than you intended, or whether your project has complications (access, structure, soil) that justify the premium. The calculator is a planning tool, not a substitute for professional quotes.