Planning permission for a granny annexe

Why the usual garden-building rules do not help

Permitted development lets you put up many garden buildings without applying, under what the regulations call Class E. The catch is in the wording: the building must be for purposes incidental to the enjoyment of the house. Government guidance gives examples such as hobbies, a home gym or storage.

A bedroom, a kitchen and a bathroom are what the guidance calls primary living accommodation. They are not incidental, so a building that contains them is outside Class E however small it is. That is why a fixed annexe nearly always needs an application.

Route one: an annexe to the main house

This is the usual route. You make a householder application, describing the building as an annexe for a family member. The council will look at the size and position, the effect on neighbours, the design and, above all, whether the annexe will really work as part of the existing home.

Councils tend to look for:

• a shared garden, drive and access, with no separate boundary drawn round the annexe;
• a size that is clearly smaller than the house;
• a position close enough to the house to be part of the same household;
• a family member as the person who will live there.

Expect a condition on the permission that the annexe stays ancillary to the main house: it cannot be sold, let or used as a separate home. That condition stays with the property after you have gone, so mention it to any future buyer.

Route two: a separate dwelling

If you want the freedom to sell or let the building later, it has to be approved as a new home in its own right. That is a full planning application, with a higher fee, tougher tests on garden space, parking and access, and in many areas a charge under the community infrastructure levy.

Many councils refuse a new dwelling in a back garden where they would happily approve an annexe of the same size. Do not apply for an annexe while privately intending to sell it off; a condition breach can be enforced years later.

Route three: the mobile-home route

Stationing a caravan in the garden of a house, and using it for purposes incidental to the house, is not development, so it does not need planning permission. The law defines a caravan by its construction and size, not by how it looks. A twin-unit caravan can be built in no more than two sections, joined on site, and must be capable of being moved in one piece once assembled.

The hard part is the use, not the size. A relative who lives entirely independently in the unit may be treated as occupying a separate dwelling, which does need permission. Where they share meals, the garden and the life of the household, the use is more likely to count as incidental. Councils judge each case on the facts.

Because of that, apply for a lawful development certificate for the proposed use before you buy the unit. It costs £274, half the householder fee, and gives you a legal record that the council agrees no permission is needed. Building regulations do not apply to a caravan, but the base and the service connections still need care.

Things that narrow the options

• A listed house: listed building consent may be needed for anything in its grounds.
• A conservation area, national park or area of outstanding natural beauty: tighter rules on outbuildings and on stationing caravans in some places.
• An Article 4 direction removing permitted development in your street.
• A condition on the original permission for an estate house, or a restrictive covenant in the deeds.
• A flat or maisonette: no householder permitted development at all.

Council tax once it is lived in

A self-contained annexe usually gets its own council tax band once someone lives in it. In England, an annexe occupied by a relative of the household is charged at half the normal rate. If that relative is 65 or over, or severely mentally impaired, or substantially and permanently disabled, the annexe is exempt.

An annexe used as part of the main home, for example as an extra bedroom for the family, also gets the 50% discount in England. Tell the council when the annexe is occupied and ask which applies.

Scotland, Wales and Northern Ireland

This guide follows the rules for England. In Scotland a fixed annexe needs planning permission and a building warrant before work starts, and the council tax rules for annexes are different. Wales and Northern Ireland have their own planning systems. In every part of the UK, the planning department of your council is the place to start, and many offer advice before you apply.