You can expect to pay around £500 - £1650 per metre square for a single storey extension. A double storey extension can cost between £500 - £1500 per meter square.Planning costs can cost between £1000 - £27000, although the average price is around £5100. Basic conservatories cost around £500 per square metre to supply and fit.If the ground needs levelling this price will increase. double storey extension can cost between £500 - £1500 per meter square. Planning costs can cost between £1000 - £27000, although the average price is around £5100.