From our local council's website:
You will not need to apply for the erection of sheds, summer houses, green houses, swimming pools, ponds, sauna cabins, tennis courts etc unless.......
You want to put up a building which is nearer to the highway than any part of the house.
More than half the area of your land is then covered by additions or buildings.
The addition or structure is to be used for anything other than domestic use.
The building is more than 3 metres high. (4 metres with a ridged roof).
The volume exceeds 10 cubic metres and is a listed building or is in any of the restricted areas. (see above)