Use this free calculator to get a realistic 2026 price range for a house extension, loft conversion or garage conversion in West London. It uses the same per-square-metre rates and loft price bands that GSM Builders Ltd quotes with across Hayes, Hillingdon, Uxbridge, Ealing, Hounslow, Harrow and Slough. Choose the project, enter the size, add any extras and you will get a low-to-high range in seconds.
How the calculator works
Extensions are priced per square metre of new floor area – £2,600–£3,600/m² for a single-storey rear extension, more per m² for a side-return (small, glazing-heavy) and slightly less for a double-storey (shared foundations and roof). Garage conversions run at £1,100–£1,700/m² because the walls and roof already exist. Loft conversions are priced as bands by type – Velux, rear dormer, hip-to-gable, L-shaped or mansard – because the structural work matters more than the floor area.
- Finish level adjusts the whole build: mid-range adds about 15%, high-end about 35%.
- Location makes a small difference within West London – Ealing, Harrow and Acton are a little higher than Hillingdon and Hounslow; Slough and Windsor a little lower.
- Extras – kitchen, bathroom, glazing, lantern, underfloor heating and structural openings – are added as their own ranges so you can see what drives the total.
- Not included: VAT at 20%, architect and engineer fees (£2,000–£4,500), council fees (£120–£258), Building Control (£800–£1,500), Party Wall surveyors if needed, and a 10% contingency we always recommend.
From estimate to fixed price
The calculator gives you a range; a site survey gives you a number. GSM Builders Ltd visits free of charge anywhere in West London, checks the drains, foundations, head height and planning position for your address, and provides an itemised written quote with agreed payment stages. Read more in our guides to house extension costs, loft conversion costs and garage conversion costs, or go straight to house extensions and loft conversions.
Calculator FAQs
How accurate is the extension cost calculator?
It uses the per-m² rates and loft bands GSM Builders Ltd quotes with in 2026, so most real quotes fall inside the range. The final price depends on ground conditions, drainage, steelwork, glazing and finishes, which is why we always survey before giving a fixed price.
Does the estimate include VAT?
No. Add 20% VAT to the range shown. Professional fees (drawings, structural calculations, planning, Building Control, Party Wall) are also excluded and typically add £3,000–£6,000.
Why is a side-return extension more per square metre than a rear extension?
Side returns are small – usually 8–12 m² – but still need foundations, steel, a glazed roof and drainage work, so the fixed costs are spread over less area.
Does the calculator cover the whole of West London?
Yes – Hayes, Hillingdon, Uxbridge, Ruislip, Southall, Greenford, Ealing, Acton, Hounslow, Harrow, West Drayton, Slough, Windsor and Maidenhead. Choose the closest area in the location box.