General advice about whether planning permission is required is on the Planning portal website.
You can find out about the planning permission required for developing your home with this simple Visual guide for householders – Interactive House/Interactive Terrace
Legislation allowing larger single-storey rear extensions to be built under permitted development rules has been laid before Parliament and will come into force on 30 May 2013. The new size limits will be subject to a neighbour consultation scheme. Details of this can be found on the Planning Portal Website
If you don't need planning permission you may require other permissions, including building regulation approval. You can get advice on these requirements by contacting us.