Under new regulations that came into effect on 1 October 2008 outbuildings are considered to be permitted development, not needing planning permission, 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
Building regulations
If you want to put up small detached buildings, building regulations will not normally apply if the floor area of the building is less than 15 square metres.
