This is a difficult question to answer because this varies depending on the area. For example, in the case of a housing development you need to look at that particular housing development's regulations and the area in which it is situated also determines whether or not this is allowed.

Permission is required for an outbuilding: In the case of an outbuilding of less than 30m² you may simply put the necessary file together yourself. If it is bigger than 30m² then you require the assistance of an architect.

Without permission: maximum one wooden summer house or one wooden pen for animals or one wooden dovecote. The construction is erected either against an existing wall that has building permission or at least 1 metre from the plot's boundaries. The surface area may be maximum 10 m². This construction may not be erected in the front garden. Cornice height is restricted to 2.5 metres, ridge height to 3 metres.

It depends on the area but normally the local municipal council can give building permission for an outbuilding.

A good site for questions like this is: www.ruimtelijkeordening.be