Granny annexe Cost Calculator
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.
Open the calculator →Price granny annexe in 30 seconds
2 steps, using the figures from our cost guide.
How this calculator works
Every figure in this calculator is the one published in the cost table on our granny annexe guide. Pick the option that matches your job and the calculator returns that published range, adjusted for your region - it does not model anything the guide does not already say.
That is deliberate. A calculator whose rates drift from the page it sits under is worse than no calculator, because two numbers on the same site disagree.
What it cannot know: access, the condition of what is already there, and anything structural. Those are what separate three real quotes from each other.
Published granny annexe costs (2026)
| Option | Typical 2026 cost | Notes |
|---|---|---|
| Compact prefab / modular | £40,000–£55,000 | |
| Mid-size modular, 1-2 bed | £55,000–£70,000 | |
| Large bespoke masonry | £80,000–£100,000 |
These are the figures from our granny annexe cost guide.
Granny annexe calculator — FAQs
It uses the same published figures as the cost guide it sits under, adjusted for your region. It is a budgeting estimate, not a quote - only a site visit produces one of those.
Related cost guides
- Granny annexe cost guide — The full price guide
- All extensions services — Browse the hub
- All BestBuilders services — Every trade we cover
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