How much would it cost to build a granny annexe?

The two guides in the chart measure slightly different things, which is why their ranges do not line up. The Garden Room Guide surveyed 176 advertised starting prices from annexe specialists in April 2026, so its figures are what a company will sell you the building for, usually before the base it sits on and the mains connections. MyJobQuote's figures are for a detached annexe built new, from the foundations to the finished rooms. Reading the two together gives you a fair idea of where a quote for your own garden is likely to land.

A one-bedroom annexe with a bathroom and kitchenSection titled A%20one-bedroom%20annexe%20with%20a%20bathroom%20and%20kitchen

For a parent who will live in the annexe full time, the usual layout is the middle band: a bedroom, a living room, a kitchen and a bathroom. The Garden Room Guide puts the building at £70,000 to £115,000, and MyJobQuote puts a new one-bedroom annexe with a bathroom and kitchen at £90,000 to £110,000. Where specialists published both a building price and a fully installed price, The Garden Room Guide found the installed one-bedroom figure came to £85,000 to £130,000.

The kitchen and bathroom are a large part of that figure. Summerhouse24, an annexe supplier, estimates in its July 2026 guide that making an annexe fully self-contained, with its own kitchen, bathroom, drainage and hot water, adds £15,000 to £30,000 over one that shares the kitchen in the main house. If your parent will eat with you most days, a bedroom, living room and shower room may be enough, and cheaper.

A two-bedroom annexeSection titled A%20two-bedroom%20annexe

A second bedroom gives a couple room to sleep apart when one of them is unwell, or gives a parent a spare room for family or for a carer to stay over. The Garden Room Guide puts a two-bedroom building at £95,000 to £155,000 or more, and MyJobQuote puts a new two-bedroom self-contained annexe at £120,000 to £150,000 or more. The Garden Room Guide also points out that price does not rise in step with floor area, because the kitchen, bathroom, connections and delivery cost much the same whatever the size, so it is worth pricing one size up before you rule it out.

1. 1

   Convert a garage or outbuilding into a bedroom and shower room

   £12,000 to £20,000 plus VAT

   The walls, roof and foundations are already there. Your parent would share the kitchen in the main house, so it is a bedroom and shower room close by rather than a separate home. Published ranges: Trade2Base £12,000 to £20,000 for a single garage with an en suite, before VAT; MyJobQuote £7,500 to £20,000 for a garage annexe conversion, without saying what that includes.
2. 2

   Convert it into a self-contained annexe with a kitchen and bathroom

   £20,000 to £70,000

   Adding a kitchen, a bathroom and their own drainage and heating is what turns a converted garage into a home. Floor levels, damp-proofing and roof height in an old garage can limit what is possible. Published ranges: MyJobQuote £20,000 to £45,000 for a one-bedroom conversion with bathroom and kitchen, £25,000 to £60,000 fully self-contained; Summerhouse24 £30,000 to £70,000.
3. 3

   Buy a self-build annexe kit and have it put up

   kits from about £25,000 to £60,000

   The lowest headline price for a new building, but a kit is the structure only. Foundations, drainage, electrics, heating and the kitchen and bathroom fit-out are all extra. Published ranges: MyJobQuote £25,000 to £60,000 for kits; Summerhouse24 kits advertised from about £30,000 to £52,000; Ideal Home kits from around £29,000 (2025).
4. 4

   Site a residential mobile home or lodge under the Caravan Act

   priced like a modular annexe

   A building made to the BS3632 residential standard, within the legal size limits for a caravan, may be sited without a full planning application when it is used as part of the household, with a Lawful Development Certificate from the council to confirm it. The saving is in planning time and possibly VAT rather than the building price. Published ranges: The Garden Room Guide and Summerhouse24 both include buildings on this route in their 2026 specialist price ranges.
5. 5

   Order a modular or prefab annexe from a specialist

   £55,000 to £115,000 for one or two bedrooms

   Built in a factory and craned or assembled on site in days or weeks. Specialist prices usually include the kitchen and bathroom, and The Garden Room Guide found many leave out groundworks and mains connections. Published ranges: BestBuilders £55,000 to £70,000 for a mid-size modular annexe; The Garden Room Guide £70,000 to £115,000 for a one-bedroom specialist annexe, before groundworks.

The cheapest route depends on what you start with. If you have a sound garage or outbuilding near the house, converting it costs less than putting up a new building in every guide above, because the walls, roof and foundations are already there. If you have no building to convert, a kit or a factory-built annexe costs less than a brick building of the same size in the guides above, though a kit leaves you to organise and pay for everything that is not in the box.

Ideal Home's guide to the cheapest way to build a granny flat makes one more point worth weighing. A bedroom with an en suite added to the house can be cheaper again, with single-storey extensions starting from around £30,000 in its 2025 figures, but your parent would share the kitchen and living room with the household rather than having a home of their own.

The mobile-home route is sometimes presented as the cheap option, but its saving is mostly in time and paperwork rather than in the price of the building. Residential lodges built to the BS3632 standard sit in the same price ranges as other specialist annexes. Where it can save money is VAT, which is covered below, and sometimes the planning application.

What a brick granny annexe costsSection titled What%20a%20brick%20granny%20annexe%20costs

A brick and block annexe, built on site from the foundations up, is the most expensive way to build one. squote, which publishes pricing guidance for builders, puts a one-bedroom brick annexe of 35 to 45 square metres at £90,000 to £140,000 finished, against £75,000 to £115,000 for the same size in timber frame. BestBuilders puts a larger bespoke masonry annexe with a full kitchen and bathroom at £80,000 to £100,000 or more. What the extra buys is a building that matches a brick house, lasts as long as it does, and keeps sound out well. If the annexe is likely to stay part of the house for good, or the planning officer wants it to match the house, brick may be worth the extra.

Timber frame and modularSection titled Timber%20frame%20and%20modular

A timber-frame annexe built on site needs lighter foundations and goes up faster than brick, which is where most of the saving comes from. A modular or prefab annexe goes further: it is built in a factory, delivered in sections and finished on site, and specialists usually sell it at a fixed price with the kitchen and bathroom included. The catch is what that fixed price leaves out. The Garden Room Guide and Summerhouse24 both found that groundwork and mains connections added £15,000 to £20,000 to specialist starting prices.

Whether a formal planning application is needed, and so whether the £548 fee applies, comes down to how self-contained the annexe is and how it will be used. 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.

Building for a parent's care needsSection titled Building%20for%20a%20parent%27s%20care%20needs

An annexe built for a parent who is getting frailer has a few extra things to settle at the drawing stage: a level way in with no steps, doorways wide enough for a wheelchair or walking frame, a walk-in shower rather than a bath, and somewhere for a carer to sleep if one may need to stay overnight. These are far simpler to build in from the start than to add once the walls are up and the bathroom is tiled. Care in a granny annexe covers how help at home can work once your parent has moved in.

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 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 drains, buried services or unstable ground can turn up on any 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, a garden building used as one does not get the size exemptions that let a garden shed or office skip this step.
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 usual routes. 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.

Time matters as well as money. Councils have a target of eight weeks to decide most householder planning applications, and that comes before any work on site. A kit-built, timber-frame or modular annexe goes up faster than a brick-and-block build, so ask each builder for a written timetable from the first day on site to handing over the keys, and build it into any plan for a parent who needs somewhere to live soon.

None of these rule out building an annexe, and for some 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.