How much would it cost to build an extension to your house
As a rule of thumb you can add 50% extra to the build cost of a single storey extension. So, Â£24,000 +50% = Â£36,000. + Add on 10-15% for professional fees (architect, planning application, building regs, structural engineer).+ VAT.rom experience, Â£1,200 per m2 is a now a more realistic estimate for a house extension cost in 2015 and reflects price increases over the last few years. So, for your 4 x 5m extension, the build cost would be roughly Â£24,000.