How much would it cost to build a granny annexe?

For most families, the annexe that makes sense for a parent is the middle band: £70,000 to £115,000 for a self-contained one-bedroom annexe with its own kitchen and bathroom. That price is for the build itself, structure, groundwork, electrics, plumbing, heating, the kitchen and bathroom fitted, and decorating throughout. The professional fees, planning application, site preparation and utility connections covered below usually sit on top of it, so treat the figures above as the building cost rather than the whole project cost.

A garage or outbuilding conversion is the cheapest route because the walls, roof and foundations already exist, and the work is mostly internal. It is usually the right choice when a parent needs somewhere close by rather than a fully separate home, but sharing a kitchen or bathroom with the main house is not the same as having their own front door. A timber-frame kit, built off-site in sections and assembled on your foundations, sits in the middle: faster to put up than a brick-and-block build and usually a little cheaper for the same size. A traditional brick-and-block annexe, built on site from scratch, costs the most but matches the main house in look and lifespan, which matters if resale value is part of the decision.

Whether a formal planning application is needed, and so whether the £548 fee applies, comes down to the same question that decides VAT: is the annexe self-contained. Do you need planning permission for a granny annexe sets out the full rule, what each type of annexe needs, and what happens if an application is refused.

Get quotes from at least three local builders before you settle on a figure. A site visit will catch the groundwork and access costs that a phone estimate cannot, and a builder who has worked on annexes in your area will have a realistic sense of what your council usually asks for.

1. 1

   Add a contingency of 10% to 15% before you agree a figure

   Before you commit

   Ground conditions are not fully known until groundwork starts, and unexpected drains, buried services or unstable ground are common rather than rare on a garden site. Treat the contingency as money held back, not spent on upgrades, until the build is under way.
2. 2

   Ask each builder for a fixed price with staged payments

   When you compare quotes

   Pay against work completed, in stages agreed before the build starts, rather than a large deposit upfront. A written, fixed-price contract protects you if material prices rise partway through the build, in a way a verbal estimate does not.
3. 3

   Building regulations approval is separate from planning permission

   Almost always needed

   It covers structural safety, fire safety, drainage, electrics and insulation, and is checked by building control as the work goes up rather than by the planning department. Because an annexe has somewhere to sleep, it falls outside the size exemptions that let a garden shed or office skip this step, even on the routes where planning permission itself is not required.
4. 4

   Decide how you will pay for it before you sign a contract

   Before you sign

   Savings, a further advance on the mortgage, equity release, or help from family are the routes most families use. A mortgage adviser, or for equity release a member of the Equity Release Council, can set out what each route costs over time before you commit to a builder.

Add the eight to thirteen weeks a planning decision usually takes to the time on site, and the two routes end up with quite different totals. A kit-built or timber-frame annexe, on site for eight to sixteen weeks once permission and building regulations are agreed, is usually four to seven months from a submitted application to moving day. A brick-and-block build, on site for four to six months, is usually six to nine months from application to moving day. Either figure is worth building into any plan for a parent who needs somewhere to live sooner than that.

None of these rule out building an annexe, and for many families it remains the right answer: a parent close by, in their own space, with the family able to see them every day. It is worth working through the four points above before you commit, because the annexe that makes financial sense is not always the annexe you pictured at the start.