Garden annexes and planning permission: where the line is drawn

Ancillary annexe or separate dwelling?

The key planning concept is whether the annexe is ancillary to the main house. An ancillary annexe is used by the household — an elderly parent, a grown-up child, guests — and shares the address, garden, parking and usually utilities. It functions as part of the home.

A self-contained unit that could be let or sold separately, with its own address, access and meters, is treated as a new dwelling. That almost always needs full planning permission and is assessed very differently.

• Who will live there and their relationship to the household
• Shared or separate access, parking and garden
• Shared or separate utilities and meters
• Whether it could realistically be let or sold on its own

Outbuildings and permitted development

Detached garden buildings can sometimes be built under permitted development as outbuildings incidental to the house. But permitted development for outbuildings does not cover primary living accommodation such as bedrooms. A garden room used as a home office may be incidental; a building people sleep in usually is not.

Size, height, position relative to boundaries and whether the property is in a conservation area or is listed also affect permitted development. Flats and maisonettes do not have the same rights.

Converting a garage into an annexe

Converting an attached or integral garage into living space often falls within permitted development if no external changes need permission. Making it an annexe with sleeping and cooking facilities raises the same ancillary question as a new building. Some newer estates have conditions removing permitted development rights, which is worth checking.

Building regulations are separate

Even where planning permission is not needed, an annexe used for sleeping needs building regulations approval — structure, insulation, fire safety and escape, ventilation and drainage. Planning decides whether you can build; building regulations decide how it is built.

The safe route: ask the council in writing

For annexes, a Lawful Development Certificate or a planning application removes the guesswork. A certificate confirms that proposed works are lawful without full permission. It is particularly useful if you later sell the house.

Councils may attach conditions to annexe permissions limiting occupation to people connected with the main house.

How we help

We build annexe-style garage conversions, outbuildings and extensions, and we raise the planning question on the first site visit. We work with your architect or planning consultant rather than guess at the answer, and we do not start until the approvals route is clear.

Common questions