A granny annexe is self contained living accommodation in, attached to or beside an existing house, occupied by someone who gives care to, or receives care from, a resident of the main house. It may be an extension, a converted garage or outbuilding, a separated part of the existing house, or a fully relocatable unit in the garden. The aim is independent living — with its own kitchen, bathroom and WC — right next to the person providing the care. The Dutch have a specific legal category for this, the mantelzorgwoning, with its own relaxed planning rules.
The planning question
There is no single answer that applies everywhere, and this is the point where most projects go wrong. The key questions are always the same:
- Is it ancillary, or a separate dwelling? An annexe used as part of the main household — shared access, shared services, not let separately — is treated very differently from a self contained dwelling with its own address. The second almost always needs express planning permission
- Where does it sit? Rules for outbuildings and extensions differ sharply between the area behind the house and anything forward of the principal elevation
- How big and how tall? Limits on footprint, ridge height and the proportion of the garden that may be covered are the usual constraints
- Is the building or the area protected? On a listed building or in a conservation area, permitted development rights are usually restricted
Always confirm with the local planning authority before you design anything. A pre application enquiry costs little and prevents a finished design from ending up in the bin.
Design and construction points
- Design it single level. Level access at the door, doorways wide enough for a walking frame or wheelchair, and a bathroom with room to turn. This is the most important decision you make and the most expensive to put right afterwards
- Escape and alarms. Interlinked smoke alarms are a basic requirement; keep the route from bedroom to outside short and unobstructed
- Services. Decide early whether the annexe runs off the existing consumer unit and meters or gets its own supply. A separate supply costs more but simplifies billing and any later independent use — and it may affect how the annexe is treated for council tax
- Drainage. The level of the existing drain often decides whether the bathroom can sit at floor level or whether you need a raised plinth or a pumped macerator. Check this before you fix the layout
- Insulation and heating. Older people sit still and want it warmer; build a well insulated envelope and use low temperature heating with underfloor pipework
- Cost. An extension or conversion into an annexe realistically runs into the tens of thousands; a turnkey relocatable unit usually sits above that and is often available on a rental basis. Get several quotations and check what is not included: foundations, connections, groundworks and fees
Related terms
- Extension
- Planning permission
- Permitted development
- Accessible design
- Adaptable housing
- Conversion
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.