What is a garage annexe?
An annexe (sometimes called a granny flat or granny annexe) is a self-contained living space that's still part of your main home. A typical garage annexe includes a bed-sitting room, a shower room with WC and a small kitchen or kitchenette, plus its own entrance. It's the most practical way to support an older relative who wants independence but needs family nearby, to give a grown-up child an affordable place to live, or to create a space that can be let to a lodger.
A double garage(around 5.5m × 5.5m) converts into a comfortable one-bedroom annexe. A detached garage with a pitched roof can often gain a mezzanine or vaulted ceiling for extra volume. A large single garage can become a studio annexe with a shower room and kitchenette if the layout is planned carefully.
Planning permission for an annexe — read this first
This is where an annexe differs from other conversions. Converting an attached garage into extra rooms for your household is usually permitted development. But creating self-contained accommodation — with its own kitchen, bathroom and entrance — can be treated as a new dwelling by the planning department, particularly if the garage is detached or if the annexe could be sold or let separately.
In practice most councils across Greater Manchester, Lancashire and Cheshire will grant permission for a genuine "ancillary" annexe used by a family member, often with a condition that it can't be occupied separately from the main house. We'll advise on the right route at your survey and, where an application is needed, prepare the drawings and submit it for you. More on planning permission.
What we include in an annexe conversion
- Full habitable-room insulation and damp-proofing to floor, walls and roof, with new windows and an insulated external door.
- Shower room with WC — new drainage connection, extractor ventilation, level-access shower where mobility is a concern, and grab-rail-ready walls.
- Kitchenette with base units, worktop, sink, hob and space for a fridge, plus the electrical supply to run it.
- Heating and hot water — either extended from the main house or an independent electric system with its own controls.
- Fire safety to Building Regulations: fire-rated separation from the main house, interlinked smoke and heat alarms, and protected escape routes.
- Accessibility where required — wider doorways, level thresholds, and layouts that work for a walking frame or wheelchair.
- Sound insulation between the annexe and the main house so both households get privacy.
Annexe conversion cost
Because an annexe includes a bathroom, kitchen and usually more structural work, it costs more than a straightforward room conversion. A double garage annexe typically costs £28,000–£45,000 depending on specification and drainage; a detached garage annexe with a new roof structure can be more. It is still a fraction of the cost of buying a separate property or paying for residential care, and it adds significant value and flexibility to your home. See our cost guide.
Council tax note: a genuinely self-contained annexe may be assessed separately for council tax, but there is a 50% discount for annexes occupied by family members and a full exemption where the occupant is a dependent relative over 65 or with a disability. We'll point you to the right guidance.