How to Build a Granny Annexe in 2026 (UK)
Building a granny annexe in 2026 starts with the planning use class, not the bricks. Whether your annexe is ancillary to your home or a self-contained dwelling decides whether you need full planning permission — and shapes everything that follows. From there it is building regulations, choosing modular or traditional build, appointing the right builder and managing the build to sign-off. Here is the full step-by-step process.
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Before you start
The first decision is the use class. An ancillary annexe used by your household may be built under Permitted Development; a self-contained annexe that could be a separate home always needs full planning permission. Get this right first — it affects planning, council tax and what you can legally do with the building.
Step-by-step: how to build a granny annexe
1. Confirm the use class
Decide ancillary vs self-contained — this determines whether you need full planning permission or a Lawful Development Certificate.
2. Secure planning and building regs
Obtain consent (or a certificate), then arrange Building Regulations approval for structure, insulation, fire safety and drainage.
3. Choose modular or traditional
Modular gives speed and price certainty; traditional gives flexibility and a match to your home. Pick based on budget and timescale.
4. Appoint a vetted builder
Compare like-for-like quotes from vetted annexe specialists and agree staged payments tied to milestones.
5. Build, inspect and sign off
Groundworks, structure and fit-out proceed with building control inspections through to the final completion certificate.
Costs and timescales
A typical granny annexe costs £45,000–£100,000+ in 2026 depending on size, build method and finish. Modular annexes can be installed in a few weeks once groundworks are done; traditional builds usually take 3–6 months. Budget for planning fees, drawings, building control and a structural warranty on top of the build.
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