estimated cost to build an extension
However, this figure has been bandied about for rather a long time now! From 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.+ Add on 10-15% for professional fees (architect, planning application, building regs, structural engineer). + VAT. Single storey extension = Â£32,000.owever, this figure has been bandied about for rather a long time now! From 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.