How Much Does a Granny Annexe Cost in 2026? (UK)
A granny annexe typically costs £40,000–£100,000 in 2026. A prefab or modular annexe usually lands at £40,000–£70,000, while a larger bespoke masonry annexe can reach £80,000–£100,000+ once groundworks and fit-out are included. Here is the full UK cost breakdown and how to keep the price down.
Granny annexe cost by type — 2026
Installed prices by build route, from this page’s 2026 figures.
| Annexe type | Typical cost | Notes |
|---|---|---|
| Compact prefab / modular | £40,000–£55,000 | the smallest modular build |
| Mid-size modular, 1-2 bed | £55,000–£70,000 | |
| Large bespoke masonry | £80,000–£100,000 | full kitchen & bathroom |
Granny annexe cost breakdown (2026)
The price of a granny annexe depends mostly on whether you choose a prefab/modular build or a bespoke masonry one, plus the size and specification. As a 2026 UK guide: a compact prefab annexe starts around £40,000, a mid-size modular build runs £55,000–£70,000, and a larger bespoke annexe with a full kitchen and bathroom reaches £80,000–£100,000+.
What affects the price
The biggest cost drivers are floor area, foundations and groundworks, utility connections (electrics, water, drainage), insulation and heating, and the fit-out of kitchen and bathroom. Difficult access, sloping plots and planning conditions can also push costs up.
How to keep the cost down
Choosing a standard modular design, keeping the footprint efficient, and siting the annexe close to existing utility runs all reduce cost. Always get three quotes from vetted specialists so you can compare both price and specification rather than headline figures alone.
How to budget step by step
1. Choose prefab or bespoke
This single decision sets your baseline budget more than any other factor.
2. Add groundworks and utilities
Foundations, drainage and connections can add several thousand pounds.
3. Factor in fit-out
Kitchen, bathroom, flooring and heating complete the all-in cost.
4. Get three quotes
Benchmark at least three vetted annexe builders before committing.
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Granny annexe — guides and detail
The detail behind the number: what changes the price, the planning position, and how to check a quote before you accept it.