Prefab & Modular Annexes
Factory-built annexes craned or assembled on site, weeks of on-site disruption instead of months, and a fixed factory price. Modular is now the fastest-growing way UK families add an annexe. We explain what quality looks like, what it really costs, and which suppliers fit which budgets.
Is this the right service for you?
This route suits families who want speed and price certainty: a parent selling their home, a care situation that can't wait a year, or a budget under £80,000. You still need planning permission and proper groundworks, but the build itself compresses from months to days.
What's Included
Speed without corner-cutting
Factory manufacture runs in parallel with your planning application and groundworks. From consent to occupation can be as little as 10-12 weeks with the right supplier.
Fixed pricing
Factory builds quote as a package: structure, fit-out, delivery and installation. Fewer surprises than a traditional build, provided the groundworks and services quote is equally firm.
Comparing specifications
Insulation values, cladding systems, window specs and build standards vary widely between suppliers at similar prices. We show you which line items actually matter.
Craneage and access checks
Modular delivery needs access: crane position, overhead cables, road width. A good supplier surveys this before you pay a deposit; walk away from any that do not.
What to check before signing a modular annexe contract
The modular annexe market has excellent manufacturers and some sheds-with-plumbing. The differences hide in the specification sheet: floor and wall U-values, whether building regulations sign-off is included, who applies for planning, and what happens to your deposit if permission is refused.
Ask every supplier the same four questions: is planning included and what happens if it fails; is building control sign-off included with certificates; is the price genuinely fixed including groundworks and connections; and can you visit a completed annexe occupied by a real family.
Payment structure matters too. Reasonable suppliers stage payments against milestones: deposit, manufacture, delivery, completion. Be wary of any structure that is majority-paid before anything arrives on site.
Common Questions
How much does a prefab annexe cost?
Entry modular studios start around £20,000-£35,000. Mid-range one-bedroom annexes run £40,000-£70,000 installed. On top, groundworks and service connections typically add £5,000-£15,000 depending on your site. Premium turnkey packages with high-spec finishes can exceed £100,000.
Do prefab annexes still need planning permission?
Yes. The construction method makes no difference to planning: self-contained residential accommodation needs consent however it is built. Some suppliers include the planning application in their package, which is worth paying for.
How long do modular annexes last?
A properly built modular annexe with structural warranties is a 60+ year building, comparable to timber-frame housing. Check the warranty: 10-year structural cover is the norm from reputable manufacturers, and lenders and buyers will ask for it.
Guides worth reading first
Related services
Discuss Your Project
Tell us about your project and we'll point you at the right route and suppliers worth talking to. No pressure, no obligation.
Get Annexe Quotes