In short - planning permission is not usually required if the following points are adhered to.
1. If you are within 2m of the boundary - the building can only be 2.5m high without planning permission - (you can have a building taller than 2.5m high close to the bounday, but you will require planning permission.)
2. If you are more than 2m from the boundary you can build up to 4m within permitted development.
3. You must not be in front of the principal elevation of the original house.
4. Outbuildings must not cover 50% of the land around the original house.
5. Any newbuilding must not be self contained living accommodation
6. Designated land may not build to the side of the house. (Designated land being Natioanl Parks, Outstanding Natural Beaty and World Heritage Sites.
7. Outbuildings are not permitted within the grounds of listed buildings.