Outbuildings are generally considered to be permitted development under Class E, and do not need Planning Permission, subject to a number of limits and conditions.
The most important factors are:
1) That the building is no higher than 2.5m from the bottom of the building to the top of the roof
2) That you do not take up more than 50% of your garden area with this or any other buildings
3) It is not to be used for living or sleeping accomodation
4) It must be situated no less than 1m from the boundary or existing house.
5) If in conservation area, planning permission must be sought regardless.
6) If installing drainage to mains for a toilet or sink a building warrant will be required.