The short answer
A fixed annexe always needs building regulations approval, because it contains sleeping accommodation. Structure, fire safety, insulation, drainage, ventilation and access all apply. A twin-unit caravan is not a building, so the regulations do not apply to it.
Planning permission decides whether an annexe can be built. Building regulations decide how. They are separate approvals, and having one does not give you the other.
Why there is no exemption for an annexe
Small garden buildings are often exempt from the regulations. The exemption is for detached single-storey buildings under 30 m² that contain no sleeping accommodation. An annexe exists for someone to sleep in, so it never qualifies, however small it is.
The one exception is the mobile-home route. A unit that meets the legal definition of a caravan is not a building, so building regulations do not apply to it. Its base, drainage and electrics still need doing properly, and the caravan should be built to the relevant standard for residential park homes.
The parts that matter most
The regulations are set out in approved documents, each covering one subject. For an annexe, these do most of the work:
- Structure (Part A): foundations suited to the ground, and a frame and roof that carry their loads. On clay near trees, this is where piles or a raft come in.
- Fire safety (Part B): smoke alarms, a safe way out from the bedroom, and limits on windows and combustible cladding close to a boundary.
- Ventilation (Part F) and energy (Part L): insulation, airtightness, heating and extract fans in the kitchen and bathroom. Some building control bodies treat a self-contained annexe as a new dwelling for energy purposes, which brings a full energy calculation. Ask early.
- Drainage (Part H): how waste water reaches the sewer, with access for rodding and a pump if the annexe sits low.
- Electrical safety (Part P): work done and certified by a registered electrician.
- Access (Part M): a level or gently ramped approach and a door wide enough for a wheelchair.
Fire engines at the end of the garden
Approved Document B expects a fire appliance to get within 45 m of every point inside a house. An annexe at the far end of a long garden, reached down a side passage, can be beyond that. The usual answer is a domestic sprinkler system, which is better to know about before the price is agreed than after.
How to apply
There are two ways to get approval in England.
- Full plans: drawings and details go to building control before work starts and are approved in advance. This is the safer route for an annexe, because problems are found on paper.
- Building notice: you give notice and the inspector approves the work as it goes. Quicker to start, but you carry the risk if something has to be redone.
You can use the council’s building control service or a registered building control approver. Either way, the inspector visits at set stages and issues a completion certificate at the end. Keep it with the house deeds; a buyer’s solicitor will ask for it.
Building control charges are set by each council and published on its website. They are a small part of the total next to the build itself, which our granny annexe cost calculator prices by size and finish.