Planning Permission may not be required subject to the following limits and conditions:
•No more than half the area of land around the original house* would be covered by additions or other buildings.
•No extension forward of the principal elevation or side elevation fronting a highway.
•No extension to be higher than the highest part of the roof.
•Maximum depth of a single-storey rear extension of 3 metres for an attached house and 4 metres for a detached house.
•Maximum height of a single storey rear extension of 4 metres.
•Maximum depth of a rear extension of more than 1 storey of 3 metres including ground floor.
•Maximum eaves height of an extension within 2 metres of a boundary of 3 metres.
•Maximum eaves and ridge height of extension no higher than existing house.
•Side extensions to be single storey with maximum height of 4 metres and width no more than half that of the original house.
•Roof pitch of extensions higher than 1 storey to match existing house.
•No verandas, balconies or raised platforms.
•On designated land* no permitted development for rear extensions of more than 1 storey; no cladding of the exterior; no side extensions.
* The term "original house" means the house as it was first built or as it stood on 1 July 1948 (if it was built before that date). Although you may not have built an extension to the house, a previous owner may have done so.
* Designated land includes national parks and the Broads, Areas of Outstanding Natural Beauty, conservation areas and World Heritage Sites
All of the above was taken from:
http://www.doineedplanningpermission.co.uk/?gclid=CNHmnLzF66wCFeIhtAodSjpjww