building an extension cost
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. Side extensions must be single storey with a maximum height of 4m. 2  If the extension is within 2m of a boundary, the maximum eaves height should be no higher than 3m. 3  A single-storey extension must not extend beyond the rear of the house by more than 3m for an attached house or 4m if the house is detached.