Multigenerational living is one of the fastest-growing renovation categories in the UK for 2026. Whether it's an ageing parent moving closer, an adult child staying longer, or simply wanting a self-contained space for income or flexibility, demand for granny annexes, garden conversions and basement suites is rising sharply.
Here's how to plan one properly — from planning permission basics to realistic costs.
What counts as a self-contained annexe?
A self-contained annexe typically has its own entrance, kitchen or kitchenette, bathroom and living space, and can function independently of the main house. This distinguishes it from a simple extension or loft conversion, which usually shares core facilities with the rest of the home. That independence is exactly what makes annexes appealing for multigenerational living — and exactly what triggers extra planning and building regulations considerations.
Popular annexe types and typical UK costs
A fully self-contained structure in the garden, separate from the house. Costs vary hugely based on size and specification, but a proper insulated, plumbed, fully liveable annexe is a significant build, not a garden room upgrade — budget accordingly.
Often the most cost-effective route if you already have a suitable structure, since the shell and foundations already exist. Insulation, plumbing and electrics are usually the main cost drivers.
The most expensive route due to excavation, waterproofing (tanking) and structural work, but often the only option where garden space is limited. Requires specialist contractors experienced specifically in basement conversions.
Planning permission: what to check first
Rules vary significantly depending on whether the annexe will be used by a dependent relative, let out separately, or sold independently in future — councils treat these differently. Key things to establish before designing anything:
- Is it within permitted development? Many outbuilding conversions and smaller garden annexes fall under permitted development rights, but size, height and proximity to boundaries all affect this — check your specific property, not general rules
- Will it be used independently or for a dependent relative? Councils sometimes attach conditions restricting an annexe's use, particularly around it being sold or let separately from the main house
- Building regulations apply regardless of planning permission — even permitted-development annexes need to meet building regs for insulation, fire safety, drainage and structural work
Speak to your local council's planning department before committing to a design, not after. A pre-application enquiry is inexpensive relative to redesigning a structure that turns out to breach a condition you didn't know existed.
Budgeting for an annexe project
Annexe projects run longer and carry more moving parts than most home renovations — groundworks, planning, building control sign-off and a full internal fit-out all happen in sequence, which means more opportunities for delay and cost creep. Build in a larger contingency than you would for a standard renovation (20-25% rather than 15-20%), and track spend by phase — groundworks, structure, services, interior fit-out — so you can see exactly which phase is running over before the whole budget does.
Plan your annexe conversion with RenovateIQ
Break the project into phases, track budget against each one, and keep every contractor's details in one place from groundworks to final fit-out.
Download Free on the App StoreA self-contained annexe is one of the biggest projects most homeowners will take on — but it's also one of the most valuable, both for family flexibility and long-term property value. Get the planning basics right early, budget generously for contingency, and track each phase separately, and it's a genuinely achievable project.