How Much Does a House Extension Cost in West London? (2026 Guide)
If you are planning to extend a house in Hayes, Ealing, Hounslow, Ruislip or anywhere else in West London, the first question is always the same: what will it cost? This guide gives you realistic 2026 figures based on the extensions GSM Builders Ltd quotes and builds every week, explains what drives the price up or down, and lists the extra costs people forget to budget for.
Want a number for your own project? Try our free extension & loft conversion cost calculator for an instant 2026 range, then book a free survey for a fixed price.
Quick answer: West London extension costs in 2026
For a standard-specification build, expect to pay roughly £2,600 to £3,600 per square metre for a single-storey extension in West London, and £2,200 to £3,000 per square metre for the upper floor of a double-storey extension (because it shares foundations and a roof with the floor below). Those figures exclude VAT, the kitchen itself, and professional fees.
| Extension type | Typical size | Build cost range (ex VAT) | Typical duration |
|---|---|---|---|
| Single-storey rear, 3 m deep | 18 to 22 m² | £50,000 to £70,000 | 8 to 10 weeks |
| Single-storey rear, 6 m deep (prior approval) | 30 to 36 m² | £75,000 to £110,000 | 10 to 12 weeks |
| Side-return (Victorian terrace) | 8 to 12 m² | £40,000 to £60,000 | 8 to 10 weeks |
| Wrap-around (side + rear) | 30 to 40 m² | £85,000 to £130,000 | 12 to 14 weeks |
| Double-storey rear | 40 to 50 m² total | £110,000 to £160,000 | 14 to 18 weeks |
| Garage conversion | 12 to 16 m² | £15,000 to £25,000 | 3 to 4 weeks |
These are the ranges we see across Hillingdon, Ealing, Hounslow and Harrow. Prices in Slough and Windsor are usually 5 to 10% lower; prices closer to central London (Chiswick, Hammersmith) are usually 10 to 20% higher.
What is included in the per-square-metre price?
A proper extension quote from a West London builder should include everything needed to get to a decorated, Building Control-certified room:
- Site set-up, skips, scaffolding and protection of the existing house
- Excavation, concrete foundations and drainage alterations
- Structural steelwork (the beam that opens up the back of the house)
- Brick or block walls, insulation, damp-proofing and the roof, flat, pitched or with a lantern
- Doors and windows, bi-folds, sliders or French doors
- First fix electrics and plumbing, underfloor or radiator heating
- Plasterboard, plastering and skimming
- Second fix electrics, lighting and sockets
- Flooring, skirting, decoration and making good
- Building Control inspections and completion certificate
What is usually not included: the kitchen units, worktops and appliances; sanitaryware if you are adding a WC; architect and structural engineer fees; planning application fees; Party Wall surveyor fees; and VAT at 20%.
The seven things that move the price most
1. Glazing
Standard uPVC French doors might cost £1,500. A 4-metre run of aluminium sliding doors can be £8,000 to £12,000, and a roof lantern adds £2,500 to £5,000. Glazing is the single biggest variable in most extension budgets.
2. Steelwork and structural complexity
Opening the whole rear wall needs a steel beam; a wrap-around extension needs two beams meeting on a post, which is more engineering and more labour. Removing internal load-bearing walls to create true open-plan space adds more again.
3. Drainage and manholes
Many 1930s West London houses have a manhole right where the extension needs to go. Building over it means a new sealed cover or diverting the drain, with Thames Water build-over approval. Budget £1,500 to £4,000 if this applies.
4. Foundations and ground conditions
Much of Hillingdon and Hounslow sits on London clay. Trees nearby mean deeper foundations (sometimes 2 to 3 m) to satisfy Building Control. A soil survey costs a few hundred pounds and can save thousands in surprises.
5. Roof type
A flat roof with a single-ply membrane is the cheapest. A pitched, tiled roof to match the house, or a vaulted ceiling with exposed rafters, costs more but looks better and adds value.
6. Finish level
Standard spec means painted walls, downlights and a laminate or engineered floor. Upgrading to underfloor heating, micro-cement floors, integrated lighting and bespoke joinery can add 20 to 40%.
7. Access
Terraced houses in Ealing and Acton with no side access mean everything. Spoil out, materials in, goes through the house. That adds labour and protection costs compared with a semi with a driveway in Ruislip or Hayes.
Other costs to budget for
| Item | Typical West London cost |
|---|---|
| Architectural drawings (planning + Building Regs) | £1,500 to £3,500 |
| Structural engineer calculations | £600 to £1,200 |
| Planning application fee (householder) | £258 (2026 fee) |
| Prior approval (larger home extension) | £120 |
| Building Control fees | £800 to £1,500 |
| Party Wall surveyor (per neighbour, if disputed) | £900 to £2,000 |
| Kitchen (mid-range, fitted) | £12,000 to £25,000 |
| Contingency | 10% of build cost |
Is an extension worth it in West London?
In most of our area, yes. Estate agents in Hayes, Ruislip and Ealing consistently report that a well-built kitchen extension adds between 10% and 15% to a property’s value, and a double-storey extension that adds a bedroom can add 15 to 20%. With average house prices in these areas between £500,000 and £750,000, a £70,000 extension frequently adds more than it costs. And you get to live in the extra space.
How to get an accurate price
Ballpark figures are useful for planning, but only a site visit gives you a real number. GSM Builders Ltd offers free, itemised extension quotes across West London. We will measure up, talk through your options, flag any drainage or foundation issues early and give you a written price broken down by stage.
Read more about our house extension service, or Related: our kitchen extension cost guide. See what we do locally in Hillingdon extensions, Hayes, Ealing, Hounslow, Ruislip and Harrow.
Thinking about timing? Read building an extension over winter in West London for the real temperature limits and a week by week programme.
Building in Uxbridge, Cowley or Ickenham? We cover the local rules and typical costs on our Uxbridge extensions page.
