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.
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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.
What drives a granny annexe’s cost
The build method is the biggest lever on price. A factory-built modular annexe arrives as a near-finished package, which is why it sits at the lower £40,000–£70,000 end of the range; a bespoke masonry annexe is built from scratch on site, takes longer and reaches £80,000–£100,000 or more once groundworks and fit-out are counted. Neither is simply better — masonry tends to add more to the value of the property and matches an existing house, while modular is faster and easier to budget. The how to build a granny annexe guide walks through both routes.
After the shell, the costs people underestimate are groundworks and services. A level, accessible plot with drainage and power nearby keeps the figure down; sloping ground, poor soil, or running a new supply and foul drainage to the bottom of the garden can add several thousand pounds before the building goes up. Fit-out spec then decides where you land: a shower room, a proper kitchen and underfloor heating cost far more than a single bedroom and a wet room.
Planning is worth settling early because it changes what you can build. An annexe used as ancillary accommodation by the same household can often go ahead under permitted development, but a self-contained home you intend to rent out, or anything in a conservation area or green belt, usually needs a full application — our guide on planning permission for an annexe covers where the line falls. If you are still weighing it up against moving or extending, is a granny annexe worth it? runs the numbers.
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.
Usually, yes. A factory-built modular or prefab annexe typically runs £40,000–£70,000 because it arrives as a near-finished package, while a bespoke masonry annexe built on site reaches £80,000–£100,000 or more. Masonry tends to match an existing house and add more to its value; modular is quicker to install and easier to budget. Groundworks and connecting services are extra on top of both.
Not always. An annexe used as ancillary accommodation by the same household — not a separate, self-contained home — can often be built under permitted development, subject to the usual limits on size, height and how much of the garden it covers. A fully self-contained annexe you intend to let, or anything on green belt or in a conservation area, normally needs a full application. Always confirm with your local planning authority before you commit.
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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