Modular granny annexes
A modular granny annexe is a small, self-contained home for a relative, built in a factory and placed in the garden or attached to the house. It shares the plot, and usually the address and utilities, with the main house.
Typical prices run from £40k to £55k for a compact studio or one bedroom to £55k to £70k for a mid-size one or two bedroom unit, before connections and groundwork extras.
Compact modular annexe
£40k to £55k
Studio or one bedroom, installed
What makes an annexe work
- It is used by family as part of the household, not let or sold separately.
- Step-free access from the drive and a level path to the main house.
- A wet room or level shower, wider doors and space to turn a wheelchair if needed later.
- Water, drainage and electricity runs that are short and practical.
- Enough privacy for both households, with a view of the garden.
- Heating and insulation good enough to be lived in all year.
Modular annexe prices
| Item | Typical price, ex VAT |
|---|---|
| Compact modular annexe, studio or one bedroom Installed, with kitchenette and shower room | £40,000 to £55,000 |
| Mid-size modular annexe, one or two bedrooms Installed, full kitchen and bathroom | £55,000 to £70,000 |
| Foundations Strip, raft, or screw piles on poor ground | £1,000 to £6,000+ |
| Planning application Householder application in England, where needed | £548 |
Where these figures come from
Typical ranges from published UK contractor and builder price guides, read November 2026, before VAT at 20%. Annexe prices are installed figures for compact and mid-size modular units; drainage and service runs vary most with the site. A guide, not a quote: the price for your house depends on a survey, the ground and the specification. Work out your own figure with the cost calculator.
Planning space for a parent or grown-up child? Tell us about the garden and we will find a specialist.
Ask for a free quoteAnnexe, extension or separate home?
The planning system cares how the annexe is used. One used by family as part of the main household is ancillary to the house. One that could be let or sold on its own is a new dwelling, which needs full planning permission and is judged much more strictly. Councils often add a condition tying the annexe to the main house.
An annexe attached to the house is assessed as an extension and may fall within the permitted development limits described on our modular extension page. A detached annexe with a bedroom and kitchen is not an outbuilding for permitted development purposes, so most need a planning application.
Mobile-home style units and the caravan rules
Some annexes are built to meet the legal definition of a caravan, so they can sit in the garden of a house without planning permission when used as part of the household. The definition limits the size and requires the unit to be capable of being moved in one or two parts. These units are not usually covered by building regulations, but councils do test whether a unit really meets the definition, so ask for a written opinion or a lawful development certificate before you buy.
Building regulations, services and council tax
A permanent annexe needs building regulations approval like any home, including fire safety, insulation and drainage. The service runs are often the hidden cost: a long trench for water, power and a drain back to the house, or a pumped drain where levels fall the wrong way.
Annexes used by a dependent relative can qualify for a council tax discount or exemption. Check with your council before you build.
From first look to moving in
Site and use
Where it goes, how it connects, who will live there and the planning route that fits.
Design and approvals
Drawings, planning or a lawful development certificate, building control.
Groundwork and build
Foundations and service trenches while the unit is made in the factory.
Delivery and connection
Craned or wheeled in, connected, tested and signed off.
Questions people ask
How much does a modular granny annexe cost?
A compact studio or one-bedroom modular annexe typically costs £40,000 to £55,000 installed, and a mid-size one or two bedroom unit £55,000 to £70,000, before unusual groundwork or long service runs.
Do I need planning permission for a granny annexe?
Usually, for a permanent detached annexe with living accommodation. An attached annexe may fall within extension limits. Units that meet the legal definition of a caravan and are used as part of the household may not need permission, but check with the council first.
Can I rent out the annexe later?
Not without permission if the council has tied it to the main house. Letting or selling it separately would usually need planning permission for a new dwelling.
Does an annexe need building regulations?
A permanent annexe does. Caravan-type units placed in the garden generally do not, but they must genuinely meet the definition.
How long does a modular annexe take?
The factory build is often six to ten weeks. With groundworks done at the same time, delivery and connection can take only days, after design and approvals.
Get prices for a modular annexe
Three short steps: where the house is, what you want to add, and how to reach you. About two minutes.
This is a free matching service, not a modular extension specialist. Once we have found a firm that builds in your part of the country, we tell you its name, and your details stay with us until you agree.
Thanks, your enquiry is in.
- We search for a firm that makes and fits the type of extension you picked, in your part of the country, then email or ring you with its name.
- Your details reach that firm only if you say yes.
- They look at the house and the access, then price the job. No marketing from us, ever.