GSM Builders Ltd designs and builds house extensions across West London – single-storey rear and side-return extensions, wrap-arounds, double-storey extensions and garage conversions. We are based in Hayes and work throughout Hillingdon, Ealing, Hounslow, Harrow and into Slough and Windsor. From drawings and planning to Building Control sign-off and the final coat of paint, one team manages the whole project.
Types of house extension we build
Single-storey rear extension
The most popular extension in West London. We take the back of the house out by 3 to 6 metres to create an open-plan kitchen, dining and family space, usually with roof lights or a roof lantern and large glazed doors onto the garden. On 1930s semis in Hayes, Hounslow and Ruislip this typically adds 20–30 m² of living space.
Side-return extension
Perfect for Victorian and Edwardian terraces in Ealing, Acton, Southall and Hanwell. We infill the narrow alley beside the original kitchen, widening the room by 1.5–2.5 m and flooding it with light through a glazed roof. Often combined with a rear extension as an L-shaped or wrap-around extension.
Wrap-around extension
A side and rear extension built as one, giving the largest possible ground-floor footprint. Ideal for semi-detached houses with a side passage. These often include a utility room, downstairs WC and a separate snug alongside the main kitchen-diner.
Double-storey extension
Adds a bedroom and bathroom above a new ground-floor room. A two-storey rear or side extension is the most cost-effective way to add a bedroom per square metre because the foundations and roof are shared. Usually needs planning permission.
Garage conversion
Converting an attached garage into a habitable room – home office, playroom, gym or a ground-floor bedroom with wet room. Involves a new insulated floor, front wall and window, wall and ceiling insulation, heating and electrics, all signed off by Building Control. See our full garage conversion cost guide.
How much does a house extension cost in West London?
Every extension is priced individually, but the table below gives realistic 2026 ranges for West London based on the projects we quote. Prices assume a standard specification, exclude VAT, and exclude the kitchen units and appliances themselves.
| Extension type | Typical size | Indicative West London cost* | Build time |
|---|---|---|---|
| Single-storey rear (3 m) | 18–22 m² | £50,000 – £70,000 | 8–10 weeks |
| Single-storey rear (6 m) | 30–36 m² | £75,000 – £110,000 | 10–12 weeks |
| Side-return | 8–12 m² | £40,000 – £60,000 | 8–10 weeks |
| Wrap-around | 30–40 m² | £85,000 – £130,000 | 12–14 weeks |
| Double-storey rear | 40–50 m² | £110,000 – £160,000 | 14–18 weeks |
| Garage conversion | 12–16 m² | £15,000 – £25,000 | 3–4 weeks |
*Indicative ranges only. Site conditions, drainage, steelwork, glazing and finishes all affect the final figure. Ask us for a free itemised quote.
Do I need planning permission for an extension?
Often not. Under permitted development a single-storey rear extension on a terrace or semi can extend 3 m (4 m on a detached house) without planning permission, and 6 m (8 m detached) if you use the larger home extension prior-approval process, where the council simply consults your neighbours. Side extensions up to half the width of the original house and single-storey are also normally permitted.
You will need a planning application for double-storey extensions, wrap-arounds that exceed the limits, any work on a flat or maisonette, and homes in conservation areas with Article 4 directions – common in parts of Ealing, Acton, Harrow on the Hill, Ruislip Village and Uxbridge. We check your address, advise on the fastest route and handle the whole application with Hillingdon, Ealing, Hounslow, Harrow or Slough council.
Building Regulations and Party Wall
Every extension needs Building Regulations approval regardless of planning. Our structural engineer designs the steel beams and foundations, and Building Control inspects at each stage – foundations, damp-proof course, drainage, structure, insulation and completion. You receive a completion certificate at the end, which solicitors will ask for when you sell.
If your extension is built on or near the boundary, or its foundations are within 3 m of a neighbour’s structure, the Party Wall Act 1996 applies. We serve the notices for you and work with the surveyors so there are no delays on site.
Our extension process
- Free site visit and quote – we measure, discuss your ideas and provide an itemised written quote.
- Design and drawings – planning drawings, structural calculations and Building Regulations drawings.
- Planning / prior approval – submitted and managed on your behalf.
- Build – foundations, drainage, structure, roof, glazing, first fix, plaster, second fix and decoration, with a dedicated site manager.
- Sign-off – Building Control completion certificate, snagging and a 12-month workmanship warranty.
Areas we build extensions in
Hayes · Hillingdon · Uxbridge · Ruislip · Southall · Ealing · Acton · Greenford & Northolt · Harrow · Hounslow · West Drayton · Slough · Windsor · all of West London
Related services
Loft conversions · Kitchen fitting · Property refurbishment · Roofing · Renovations
Start your extension
Send us a sketch, a Pinterest board or just a description of what you need. We will arrange a free site visit anywhere in West London and provide a clear written quote.
