Cost guides · 5 min read
How much does a granny annexe cost?
Multigenerational living is quietly booming: parents moving closer, adult children saving for deposits, or simply keeping family near without sharing a kitchen. An annexe gives independence on one plot, and there are three ways to build one.
The three annexe routes
| Route | What it involves | Indicative cost (2026) |
|---|---|---|
| Garage conversion annexe | Convert an attached or detached garage with shower room and kitchenette | £20,000 to £45,000 |
| Garden annexe | A self-contained insulated garden building with bathroom and kitchenette | £35,000 to £70,000 |
| Extension annexe | A ground-floor extension with bedroom, wet room and living space | £60,000 to £120,000+ |
The planning rule that catches people out
Planning treats an annexe used as part of the main household very differently from a separate dwelling. A granny annexe that shares the plot and is ancillary to the house (family using the same garden, no separate address) is usually acceptable, and often possible under permitted development for conversions. Building a fully independent home in the garden is a different application entirely. We design to the right side of that line and handle the council conversation.
What makes an annexe genuinely work
- Level access and a wet room rather than a bathtub, futureproofing from day one.
- Proper heating and insulation, an annexe is a home, not an outbuilding.
- Sound separation from the main house for everyone’s sanity.
- A kitchenette sized to real use: fridge, hob, sink, storage.
Worth knowing
An annexe for a dependent relative can qualify for council tax exemptions or discounts in many boroughs, and a well-built annexe adds flexible value: home office today, teenager’s space tomorrow, rental potential later where rules allow.
Frequently asked questions
Do I need planning permission for a granny annexe?+
Garage conversions and modest garden annexes used as part of the household often fall under permitted development, but the ancillary-use rules matter and councils differ. We check your plot and handle the application where needed.
Can an annexe have its own kitchen and bathroom?+
Yes, a kitchenette and wet room are standard. It is the separate-dwelling test (independent address, severed from the household) that changes the planning position, not the kitchen itself.
Which annexe route is best value?+
If you have a garage you rarely use, converting it is by far the cheapest route to independent space. No garage, a garden annexe usually beats an extension on cost, though an extension integrates best for accessibility.