Annexe extensions

An annexe extension creates a self-contained area attached to your home, typically with a bedroom, an accessible shower room and a small kitchen or kitchenette, so an older relative, adult child or carer can live close by with their own space.

Planning treats annexes carefully. An annexe used by members of your household, connected to the main house, is normally assessed as an extension; one that could be let or sold separately is treated as a new dwelling. We design and describe it correctly from the start.

What is included

• Design for accessibility: level thresholds, wider doors, walk-in shower
• Planning application describing ancillary use
• Separate or shared heating and hot water
• Kitchenette, shower room and bedroom finish
• Fire separation and alarms to building regulations

Who it suits

• Multi-generational families
• Relatives who need support close by
• Homes with a large side or rear garden

Design advice

Accessibility is cheaper to design in than to retrofit. Level access, a 900 mm door width, a level-access shower and space to turn a wheelchair can all be planned into the footprint for little extra cost.

How much does it cost?

National guide prices including VAT at 20%. Build cost only; design, structural and council fees are extra. Your area page shows prices adjusted for your region.

Planning rules across the UK

England

In England an annexe that stays part of the household and connects to the house is assessed as an extension, but its size usually takes it beyond permitted development, so a householder application describing the ancillary use is the normal route.

Wales

In Wales an annexe linked to the house and used by your household is assessed as an extension, but its size usually means a planning application is needed.

Scotland

In Scotland, householder permitted development rules (Class 1A for single-storey ground-floor extensions and Class 1B for extensions of more than one storey) allow some smaller extensions without a planning application, within limits on depth, height and distance from the boundary. Conservation areas and listed buildings are more restricted. Separately, almost every extension needs a building warrant from the council before work starts.

Northern Ireland

In Northern Ireland the Planning (General Permitted Development) Order 2015 lets some single-storey rear extensions go ahead without planning permission, up to 3 m on a semi-detached or terraced house and 4 m on a detached house, within height and boundary limits. Larger, side and two-storey extensions normally need planning permission from the council, and building control approval is separate.

Building regulations and neighbours

Building regulations approval is separate from planning. In England you can use the council's building control team or a registered building control approver; a full plans application gets the drawings checked before work starts, and the inspector signs the work off with a completion certificate.

In England and Wales, if the work involves a shared wall, building on the boundary, or digging foundations within 3 m (sometimes 6 m) of a neighbour's building, the Party Wall etc. Act 1996 applies and notice must be served before work starts: two months for work to a party wall, one month for new walls on the boundary and for excavations. Scotland and Northern Ireland have no Party Wall Act.

Questions about annexe extensions