There are two Local Authority requirements before you begin your project. The first of these is Planning Permission, which is whether or not you will be allowed to build it. Extensions and major alterations to your existing house will usually require planning approval from your local authority planning department.
However, some projects do not require planning permission. For example, Planning Permission may not be needed if the extension is single-storey, to the rear of your existing property, and less than 50 cubic metres in volume.