Do I need planning permission for a granny annexe in London?

The short answer: three kinds of garden building

Planners sort every building in the garden into one of three kinds, and an annexe sits in the middle one. The first is an incidental building: a shed, a home office, a gym. Planning law says using buildings within the curtilage of a house for any purpose incidental to the enjoyment of the dwellinghouse is not development at all, and Class E of the permitted development order lets you put up such a building within size limits.

The second is an ancillary annexe: real living accommodation, used by someone who is part of the household, such as a parent who eats with the family and shares the garden and the front door. The third is a separate dwelling, a self-contained home that happens to stand in your garden. The middle kind needs a planning application; the third needs one too, and is judged as a new house. Our outbuilding planning guide covers the size limits for the first kind, so this post deals with the step beyond it.

Why Class E stops short of an annexe

Class E is the right most garden buildings rely on, and the government's technical guidance draws its edge clearly. It says buildings under Class E should be built for purposes incidental to the enjoyment of the house, and that a purpose incidental to a house would not, however, cover normal residential uses, or the use of an outbuilding for primary living accommodation such as a bedroom, bathroom, or kitchen.

So the size of the building is not the test. A small cabin with a bed, a shower and a hob is outside Class E, while a large garden office with a WC can sit inside it. This catches families who order a garden room under permitted development and later move a parent into it: the building may have been lawful as an office, but councils usually treat the change to living accommodation as needing permission. Our post on garden room drawing costs covers the incidental route if the building will stay an office or studio.

Ancillary annexe or separate home: what the council looks at

The legal line between an annexe and a second house comes from the same section of the Planning Act, which says using as two or more separate dwellinghouses any building previously used as a single dwellinghouse involves a material change in the use. An annexe that stays part of the one household is not that change. One that works as its own home is.

There is no single statutory checklist, so officers look at how the plans and the way of life fit together. Signs of an ancillary annexe are a shared garden and driveway, a path to the main house, shared meter or drainage connections, and an occupant who relies on the family. Signs of a separate dwelling are its own front door to the street, a fenced-off garden, its own postal address, and a layout that would suit a tenant. We draw the annexe so the plans show the shared links honestly, because a design that looks lettable invites a refusal or a condition you did not expect.

Councils often grant annexes with an occupancy condition. The Act lets a council impose conditions for regulating the development or use of any land under the control of the applicant, and the usual wording keeps the annexe in use ancillary to the main house and stops it being sold or let as a separate home. Read that condition before you build: it affects how you can use the annexe if your family's needs change.