Cost guide · 2026
HouseExtensionCostinLondon2026
Single-storey, double-storey, side return and wraparound — full 2026 cost guide for London house extensions based on completed projects across all 33 boroughs.
Quick answer
A house extension in London costs £50,000–£220,000 in 2025–2026. A side return runs £50,000–£90,000; a single-storey rear extension £60,000–£140,000; a wraparound £95,000–£170,000; a double-storey £110,000–£220,000. Cost per m² is £2,800–£4,200. Kitchen units, appliances and worktops are typically budgeted separately. All Builderr quotes are fixed-price.
What moves the price
The seven cost factors that matter most.
Side return £50–90k; rear £60–140k; wraparound £95–170k; double-storey £110–220k.
Each metre of rear depth adds £8–14k once past 3m.
Bifolds vs sliders £4–8k delta; lantern £4.5–8.5k; steel-framed Crittall +£6–12k vs standard aluminium.
Each load-bearing wall removed +£3,500–£6,500 (steel, padstones, MS work).
Mid-spec kitchen £15–30k; bespoke £30–60k. Quote separately.
PD £2,500 fees; full planning £6,500 + 8–12 weeks; appeal £12,000 + 6 months if refused.
Where existing foundations are insufficient, +£18–35k. Always surveyed before contract.
Cost breakdown
Where the money actually goes.
Percentage breakdown of a typical mid-spec project. Add or subtract per your finish tier.
| Line item | % of total | £ on avg project |
|---|---|---|
| Groundworks & foundations | 12% | £16,200 |
| Superstructure & roof | 15% | £20,250 |
| Glazing | 13% | £17,550 |
| First fix MEP | 9% | £12,150 |
| Plaster & decoration | 7% | £9,450 |
| Kitchen fit-out | 9% | £12,150 |
| Flooring | 6% | £8,100 |
| Second fix & lighting | 4% | £5,400 |
| External works | 3% | £4,050 |
| Fees | 10% | £13,500 |
| Site management | 6% | £8,100 |
| Margin & contingency | 6% | £8,100 |
Real projects
Worked examples from completed builds.
London zones
Where your borough sits in the price band.
| Zone | Multiplier | Notes |
|---|---|---|
| Zone 1 | 1.20–1.28× | Access, conservation, listed common |
| Zone 2 | 1.10–1.16× | Premium spec common |
| Zone 3 | 1.00–1.08× | Baseline |
| Zone 4 | 0.93–1.00× | Mainly semi stock |
| Zone 5–6 | 0.85–0.93× | Detached and bungalow stock |
Builderr vs other London builders.
The construction industry has a wide distribution of operators. Here's what changes between a directly-employed, fixed-scope outfit and the alternatives.
| Criterion | Builderr | Typical London builder | Cowboy outfit |
|---|---|---|---|
| Labour model | Directly employed team (PAYE) | Mixed subcontract gangs | Day-rate cash labour |
| Pricing | Fixed-scope itemised quote | Estimate + provisional sums | Verbal price + variations |
| Design & engineering | In-house architect + SE | Outsourced, separate billing | Builder draws on the back of an envelope |
| Planning + LDC handled | Yes — included in price | Often charged extra | Builder asks you to apply |
| Party wall surveyors | Instructed by us | Your responsibility | Skipped (illegal) |
| Building control | Plans + site inspections booked by us | Building Notice route | Not registered |
| Project management | Dedicated PM, weekly photo updates | Foreman doubles up | Owner-manager juggles 5 jobs |
| Payment schedule | Stage payments against signed-off milestones | Weekly invoices | Cash up front |
| Insurance | £10M PL + 10yr structural warranty | £2–5M PL only | No documented cover |
| Snags at handover | <3 typical | 20–30 typical | Walk-off mid-job common |
| Variation creep | 0% — fixed scope | +15–25% over original quote | +40%+ regularly |
Save £27,000–£60,750 on a house extensions.
Industry data (FMB, RICS, Which? Trusted Trader 2024) shows the average London construction project overruns by 18–22% on cost and 25–35% on time. Fixed-scope contracts with a single accountable team eliminate that variance. The savings above assume a typical project at £135,000.
Related
See the full house extensions service page.
FAQ
House Extension Cost in London:
common questions.
How much does a rear extension cost in London?
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A 4m single-storey rear extension in London costs £60,000–£110,000; a 6m rear extension £85,000–£140,000. This is the building work only — kitchen units, appliances and worktops add £15,000–£50,000 depending on spec. Inner London adds a 10–20% zone uplift.
How much does a side return extension cost?
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A side return extension costs £50,000–£90,000 depending on length and spec. It is typically the cheapest extension type because it infills an existing narrow alley rather than projecting into the garden — less new structure, shorter span steels, simpler drainage. Most are permitted development.
How much does a double-storey extension cost in London?
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A double-storey rear or side extension in London costs £110,000–£220,000. The additional cost over single-storey comes from: deeper foundations, first-floor structural design, roof works, and typically an ensuite or bedroom fit-out on the upper level. Double-storey extensions always require full planning permission.
What is the cost per m² for a house extension in London?
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House extensions in London typically cost £2,800–£4,200 per m² for the build work. This includes structure, roofing, glazing, MEP first fix, plaster and decoration, and basic flooring. It excludes kitchen units, appliances, bespoke finishes and fees (architect, structural, planning). Zone 1 inner London: add 20–28%.
Does a house extension cost include kitchen?
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Builderr's extension quotes include the building work but exclude kitchen units and appliances — these are supplied and installed as a separate package because clients have widely varying kitchen specs (£15,000–£60,000 range). The extension quote includes all structural work, glazing, MEP first fix, flooring preparation and basic kitchen connection points (electrics, plumbing, drainage).
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