Cost guide · 2026
HouseExtensionCostWestLondon2026
2026 house extension pricing for West London — Ealing, Hounslow, Hammersmith & Fulham, Brent, Harrow, Richmond, Kensington & Chelsea. Edwardian semis, Victorian terraces, and the most expensive postcodes in the city.
Quick answer
A house extension in West London costs £58,000–£260,000 in 2026. Side returns £58,000–£100,000; single-storey rears £68,000–£165,000; wraparounds £110,000–£195,000; double-storeys £125,000–£260,000. Kensington & Chelsea and prime Hammersmith carry premium pricing — listed buildings, conservation, premium spec. Outer Hounslow, Ealing and Harrow more competitive. Cost per m² is £3,100–£4,800.
What moves the price
The seven cost factors that matter most.
K&C, prime Hammersmith — steel Crittall, hand-built joinery, MVHR baseline. £180–260k typical for 4-6m rear with full prime spec.
Ealing, Chiswick, Brondesbury — generous gardens, 4-6m rears with open-plan kitchen at £105–155k.
Parsons Green, Munster Village, Brackenbury — side returns dominant at £75–110k including kitchen.
Bedford Park, Brook Green, Holland Park, Notting Hill, Brackenbury — full planning standard, +£6,500 fees, +10 weeks.
Listed building consent typical in Chelsea, South Ken, Brompton — adds 12–20 weeks and £2,500–£6,500 in heritage statement and consent fees.
Hounslow (TW3-TW8), Harrow (HA1-HA9) — easier access, lower trade rates, 12–18% below inner West.
Inner West default: steel Crittall £18–28k or premium slider £14–22k. Outer: bifold £8–12k standard.
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 | 11% | £17,490 |
| Superstructure & roof | 15% | £23,850 |
| Glazing | 15% | £23,850 |
| First fix MEP | 9% | £14,310 |
| Plaster & decoration | 7% | £11,130 |
| Kitchen fit-out | 8% | £12,720 |
| Flooring | 6% | £9,540 |
| Second fix & lighting | 5% | £7,950 |
| External works | 3% | £4,770 |
| Fees | 12% | £19,080 |
| Site management | 7% | £11,130 |
| Margin & contingency | 2% | £3,180 |
Real projects
Worked examples from completed builds.
London zones
Where your borough sits in the price band.
| Zone | Multiplier | Notes |
|---|---|---|
| Prime (SW3, SW10, W8, W11) | 1.22–1.40× | Chelsea, South Ken, Kensington, Notting Hill — listed common |
| Inner West (W2, W4, W6, W12, W14, SW6) | 1.12–1.22× | Paddington, Chiswick, Hammersmith, Shepherds Bush, Fulham |
| Mid West (W3, W5, W7, W9, W10, W13, NW6, NW10) | 1.00–1.12× | Acton, Ealing, Hanwell, Maida Vale, Brent |
| Outer West (TW3-TW10, HA1-HA9, UB1-UB8) | 0.90–1.00× | Hounslow, Harrow, Hillingdon — semi-detached 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 £31,800–£71,550 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 £159,000.
Related
See the full house extensions service page.
FAQ
House Extension Cost West London:
common questions.
How much does a house extension cost in West London?
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A West London house extension costs £58,000–£260,000. Side returns on Hammersmith and Fulham terraces £78,000–£110,000. 4m rears on Ealing and Chiswick Edwardian semis £105,000–£155,000. Wraparounds with steel Crittall on prime terraces £150,000–£210,000. Prime K&C double-storey projects with hand-built joinery and listed building consent reach £230,000–£260,000+.
Why are Kensington & Chelsea extensions so expensive?
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K&C combines premium factors: extensive conservation areas (Chelsea, South Ken, Brompton, Holland Park), high concentration of listed buildings (listed building consent required), premium spec expectations (steel Crittall £18–28k, hand-built joinery, MVHR), narrow streets with severe access and parking constraints, and trade rates 25–35% above outer London. A wraparound with full prime spec runs £180,000–£260,000.
Which West London borough is cheapest for an extension?
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Outer Hounslow (TW3, TW4, TW5, TW7), outer Harrow (HA2, HA3, HA4, HA7) and Hillingdon (UB1-UB8). Lower trade rates, larger gardens enabling bigger PD extensions, easier vehicle access, minimal conservation pressure. A 4m rear on a 1930s semi typically £82,000–£115,000 vs £130,000–£175,000 in inner Hammersmith or prime K&C.
Do Edwardian semis in Ealing suit big extensions?
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Yes — Edwardian semis in Pitshanger, North Ealing, Brentham and Hanwell typically have generous gardens (often 30m+ depth) and side-access alleys that suit both rear and wraparound extensions. A 6m rear with open-plan kitchen typically £125,000–£170,000; a wraparound £155,000–£200,000. Most are permitted development outside the small Brentham conservation area.
How long does a West London extension take?
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12–24 weeks on site. Side returns 12–14 weeks; 4m rears 15–17 weeks; wraparounds 18–20 weeks; double-storeys 20–24 weeks (prime spec at upper end). Pre-construction is 4–6 weeks for PD in outer Hounslow/Harrow, 14–20 weeks for full planning in K&C conservation or listed building consent.
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