London extension cost calculator
What a London extension, loft or basement really costs, with fees, VAT and contingency.
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Most people price an extension on the build alone and get a shock when the fees, VAT and contingency land on top. This calculator gives you the full figure for a London project, adjusted for your borough.
Prime central boroughs such as Westminster, Kensington and Chelsea and Camden usually cost around 10% more to build in. Access is harder, skips need permits and trades charge more. Outer boroughs such as Bexley, Havering and Sutton tend to come in under the London average.
The build rates are per square metre of new floor area and assume a typical London terrace or semi. A kitchen, glazing and structural steel can swing the cost a long way, so treat the range as a starting point for talking to an architect and a builder.
Questions people ask
How much does a single storey rear extension cost in London?
For a good standard finish, expect around £3,000 to £3,500 per square metre for the build in an average London borough, before VAT and fees. A 25 m² rear extension usually comes to £110,000 to £140,000 all in once VAT, professional fees and a 10% contingency are added.
Do I pay VAT on an extension?
Yes. Extensions and loft conversions to an existing home are standard rated at 20%. Some builders below the VAT threshold won't charge it, but most firms working in London will.
What fees should I budget for on top of the build?
Allow for an architect (often 8% to 15% of build cost), a structural engineer, building control, a party wall surveyor if you share a wall, and the planning fee if you need permission.
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