Jefner has covered off the Building Control side, but really this is a Planning issue. These are the situations when a Planning Application must be made for an outbuilding. (An outbuilding is only an outbuilding if ALL parts of it are more than 5 metres from the original house).
You want to put up a building or structure which would be nearer to any highway than the nearest part of the "original house", unless there would be at least 20 metres between the new building and any highway. "Highway" includes public roads, footpaths, bridleways and byways.
More than half the area of land around the "original house" would be covered by additions or other buildings.
The new structure is not to be used for domestic purposes. For example, for parking a commercial vehicle, running a business or for storing goods in connection with a business.
The structure is more than 3 metres high, or more than 4 metres high if it has a ridged roof. (Measured from the highest ground next to it.)
If the main house is a listed building, and you want to put up a structure with a volume of more than 10 cubic metres.
If the main house is in a Conservation Area, a National Park, an Area of Outstanding Natural Beauty, or the Norfolk Broads, and you want to put up a structure with a volume of more than 10 cubic metres.