How to Build a Granny Annexe in 2026 (UK)

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 insured 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.

FAQs

Related guides

• Do I need planning permission for an annexe?
• Compare granny annexe builders
• How-to guides
• Is it worth it? Insights