Extension cost calculator for London
What a house extension costs to build in London in 2026, before you have a single quote. Pick the type, size and specification, tick the extras, and get a low–high build cost range with every line itemised. Then add the professional fees for the whole-project figure.
Guide prices, London, ex VAT, 2026. Not a quote. Build cost only — no fees, VAT or contingency. Rates come from published London cost guides listed underhow we worked out these figures.
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What moves the cost of a London extension
Specification
The biggest single variable. Published London build-only rates, ex VAT and fees, run at about £2,800–£3,200 per m² for a basic specification, £3,200–£4,200 for mid-range and £4,200–£5,500+ for high-end. Two extensions on the same footprint can be tens of thousands apart purely on glazing, kitchen and finishes.
Type of extension
A side return costs more per m² than a plain rear extension (fixed costs over a small footprint, tight access); a wraparound costs more again (more steel, a corner junction and usually full planning); a double-storey is the best value per m² because foundations and roof serve two floors. The calculator applies each difference as published by London sources — see the method below.
Size
Fixed costs — a steel beam, drainage connections, building control, fees — are paid whether the extension is 12m² or 30m², so small extensions cost more per square metre than the averages suggest.
Extras outside the base rate
- Kitchen: published at £15,000 to over £50,000 — the line most often underestimated.
- Bifold or sliding doors: aluminium sets at around £3,150–£7,200 supplied and fitted.
- Roof lanterns or structural rooflights: around £2,000–£5,000 each.
- Wet underfloor heating: about £70–£140 per m² in London in a new screed floor.
What sits outside this calculator
Professional and statutory fees, VAT, contingency, and the risks specific to your house — deeper foundations near trees or on clay, a drain in the way, restricted access. Therenovation fees calculator adds the fees; thecontingency guide covers the buffer; and once a builder has priced it, the overrun calculator shows where that price is likely to land. For the wider picture, readhouse extension and loft conversion costs in London.
How we worked out these figures
The per-m² rates for each specification are Build Team’s published 2026 London build-only rates, ex VAT and fees. We adjust them by extension type using the ratios in other published London price guides: side return ×1.09 at the low end and ×1.06 at the high end, and double-storey ×0.91 / ×0.88 (both from EMA Structures’ standard-spec rates against its rear extension rate); wraparound ×1.22 / ×1.28 (from Draw & Plan’s type rates). Underfloor heating uses BestBuilders’ UK rate with the source’s own 15–25% London uplift applied. Totals are rounded to the nearest £1,000. Sources checked on 27 September 2026.
- Build Team — How much does an extension cost in London? 2026 guide — London build-only, ex VAT and fees: basic £2,800–£3,200/m²; mid-range £3,200–£4,200/m²; high-end £4,200–£5,500+/m². Kitchen £15,000–£50,000+; aluminium bifolds £3,150–£7,200; roof lantern/structural rooflight £2,000–£5,000 each; structural engineer £1,500–£4,000.
- EMA Structures — House extension cost London 2026 — Standard spec, ex VAT: single-storey rear £2,200–£3,400/m²; side return £2,400–£3,600/m²; two-storey £2,000–£3,000/m². Rear extension structural package £1,400–£3,500 + VAT.
- Draw & Plan (London) — house extension drawings — Planning + Building Regulations drawings from £1,650 + VAT including measured survey. 2025 per-m² estimates by extension type (rear £1,800–£2,500; wraparound £2,200–£3,200).
- Builderr — House extension cost in London 2026 — Cross-check: £2,800–£4,200 per m² build work, excluding kitchen units and fees.
- BestBuilders — Underfloor heating cost UK 2026 — Wet UFH in new-build/extension screed £60–£110/m² supplied and installed, ex VAT; "London and the South East add roughly 15–25%".
Some London sources publish lower rates than the ones used here — EMA Structures’ standard specification is £2,200–£3,400 per m² — so a simple, well-drawn rear extension with good access can come in below the calculator’s range. Guide prices, London, ex VAT, 2026. Not a quote.
Common questions
How much does an extension cost per square metre in London in 2026?
Published London build-only rates, ex VAT and fees, run at about £2,800–£3,200 per m² for a basic specification, £3,200–£4,200 for mid-range and £4,200–£5,500 or more for high-end (Build Team, 2026). Some London firms publish lower standard-spec figures — EMA Structures quotes £2,200–£3,400 per m² for a single-storey rear extension — so treat the lower end of any range as achievable only with a simple design, good access and firm drawings.
How much does a 20m² single-storey extension cost in London?
At a standard specification the published London rate of £3,200–£4,200 per m² gives roughly £64,000–£84,000 to build, ex VAT, before professional fees and before a kitchen. Add 20% VAT if your builder is VAT-registered. Professional and statutory fees come on top — the renovation fees calculator itemises them.
Is a side return extension cheaper than a rear extension?
Usually cheaper in total because it is smaller, but more expensive per square metre. Fixed costs — a steel beam, drainage, building control, fees — are spread over fewer square metres, and access down the side of a terrace is tight. EMA Structures publishes side returns at about 6–9% more per m² than a plain rear extension, and the calculator applies that difference.
Is a double-storey extension better value?
Per square metre, generally yes: the foundations and roof — the two most expensive elements — are shared across two floors. EMA Structures publishes two-storey extensions at about £2,000–£3,000 per m² against £2,200–£3,400 for single-storey. The total is still higher because you are building more floor area, and a double-storey addition almost always needs full planning permission.
Do these extension costs include VAT?
No. Every figure on this page is ex VAT. A new extension to an existing home is standard-rated at 20% where the builder is VAT-registered, so a £75,000 build becomes £90,000. Always check whether a quote shows VAT before comparing it with another.
What does the extension cost calculator leave out?
Professional and statutory fees (architect, planning, structural engineer, party wall, building control, project management), VAT, a contingency, and anything you have not ticked — kitchen, glazing upgrades and underfloor heating are optional extras. Use the renovation fees calculator for the fees and hold a contingency on top.
Next step: renovation fees calculator. Going up instead? loft conversion cost calculator. Collecting quotes: builder quote comparison checklist · how to compare builder quotes. The whole budget: budget planner · all resources. Before you start: planning permission checker · timeline calculator.
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