Garden annexe rules: planning permission, council tax and building regulations

What legally counts as an annexe

Planning officers look for independent living: sleeping space, cooking facilities and washing facilities combined in a way that allows someone to live there without using the main house. A garden office with a WC and a kettle is an incidental outbuilding. Add a bed, a shower and a full kitchen and it is ancillary accommodation, which needs planning permission.

Planning permission and the ancillary test

Most annexes are approved as ancillary to the main dwelling — occupied by a family member, sharing the same address, not sold or let separately. Councils often attach a condition preventing separate occupation. A separate, self-contained dwelling is a much bigger application, assessed on access, parking, amenity and local policy.

Building regulations

Unlike a garden office, an annexe is habitable and therefore fully notifiable. Expect a building control application covering structure, fire safety and escape, insulation, ventilation, drainage and electrical certification. Budget for the inspection fees and a slightly longer programme.

• Interlinked mains-powered smoke alarms
• Escape window or protected route from any sleeping space
• Part L insulation and Part F ventilation compliance
• Connection to the foul drainage run, or an approved pump station

Council tax and letting it out

A self-contained annexe can be banded separately for council tax by the Valuation Office. Where it is occupied by a dependent relative, or used as part of the main home, a discount or exemption often applies — check with your local authority before you build.

Renting an annexe to a third party is a different use class and usually needs its own planning consent, plus the standard landlord obligations: EPC, gas safety where applicable, electrical inspection and deposit protection.