A building in the grounds of a dwellinghouse but that is not physically attached to it will not require planning permission provided:
- It is for a purpose incidental to the existing house, for example, for storage, a games room, gym or indoor swimming pool. It must not be for a use that forms one of the main functions of a house such as a kitchen or bedroom, or for use as a separate dwelling (occupied by someone who is not part of the household), or for a business use (unless it is only a resident of the house who uses it and no disturbance is caused to the surrounding area: see To Run a Business from Home https://planningaid.zendesk.com/hc/en-us/articles/203144352-To-run-a-business-from-home for more details);
- the total area of ground covered by additional buildings (other than the original house) would be more than 50% of the total area of the grounds of the house (excluding the ground area of the original house);
- It is not forward of the principal elevation of the house;
- It is single-storey;
- It is not higher than
- 4 metres if it has a roof that slopes in more than one direction;
- 2.5 metres if it is within 2 metres of the boundary (regardless of roof shape);
- or 3 metres in any other case;
- The eaves (where the roof meets the walls) are not more than 2.5 metres high;
- It is not within the curtilage of a listed building;
- It does not include a verandah, balcony, raised platform or microwave antenna;
- The capacity of any container would exceed 3,500 litres;
- If the property is in on Article 2(3) land, i.e. a National Park, an Area of Outstanding Natural Beauty, a Conservation Area, an area designated under s41(3) of the Wildlife and Countryside Act 1981, the Norfolk Broads or a World Heritage Site, it is not between a side elevation of the house and a boundary.
If the dwelling is situated in a World Heritage Site, a National Park, an Area of Outstanding Natural Beauty or the Norfolk Broads, the total ground area of outbuildings must not exceed 10 square metres.
See also Property Alteration Overview for more potential restrictions: https://planningaid.zendesk.com/hc/en-us/articles/203210341-Property-Alterations-Overview