A garden room used for purposes incidental to the enjoyment of the house is generally exempt within the limits for domestic sheds and structures — floor area, height, position behind the house and not reducing private open space below the minimum.
The key test is incidental use. A home office, gym or store used by the household is incidental. A structure with a bedroom, kitchen and bathroom used as separate accommodation, or a business with staff and visitors, is not.
The exemption also requires that the structure is to the rear of the house, within the height limits for pitched and flat roofs, and that at least 25 square metres of private open space remains.