Question: how much does it cost to build an extension per square metre?
Answer: The average cost per m2 for an extension outside of London is between £1,200-£1,500. In London and the South East, the average cost per m2 is around £1,500-£2,000+. For two-storey extensions, add 50% to the cost of a single-storey extension. If you want to add a bathroom or kitchen, add £5,000-£10,000.