average cost of 1 storey extension
Extensions are more expensive: RICS gives a guide cost of between Â£1,260 and Â£1,680 per square metre for an extension with plastered finish. So a 20-square metre single-storey extension will range from Â£25,200 to Â£33,600 â before you've put down flooring or installed a kitchen.s a rough guide, the Royal Institution of Chartered Surveyors (RICS) suggests that the cost of a loft conversion for a plaster-finished shell, excluding decoration and floor finishes, is Â£660 to Â£900 per square metre including VAT.