Granny Annexe Size Limits: How Big Can You Build?
Once families understand that most granny annexes don't need full planning permission, the next question is usually: so how big can we actually go? It's a fair question — the size of your annexe affects everything from layout and number of bedrooms to cost and build time. Here's how the size rules actually work.
We build granny annexes and garden annexes for families across Essex and Hertfordshire, as well as the rest of the UK, and getting the size right — both what's legally possible and what genuinely suits your family — is one of the most important parts of the design conversation.
Why size matters legally
As covered in our previous posts, most granny annexes are built under the Caravan Act, which classes them as a type of mobile home rather than a permanent structure. This is what allows most annexes to avoid full planning permission, needing only a Certificate of Lawful Development instead. But that classification only holds if the annexe meets specific size and construction rules — go beyond them, and you may need to apply for full planning permission instead.
The maximum size
As a general guide, granny annexes built under the Caravan Act can go up to 20m (65ft) x 6.8m (22ft), giving a maximum of 136m². That's a substantial amount of space — enough to comfortably design anything from a compact studio through to a full one-to-five bedroom annexe, depending on your garden and requirements.
It's worth noting that the exact rules and how they're interpreted can vary slightly by council, so the specific specification needs to be checked against the current rules for your plot and local authority — which is exactly why a proper site survey and planning conversation should be step one of any project.
What does 136m² actually look like?
To put that in context, 136m² is roughly comparable to a good-sized three or four-bedroom house. Within that overall limit, you have real flexibility in how the space is configured:
Studio/one-bedroom annexe — a compact, efficient layout for a single person or couple, using a fraction of the maximum allowance.
Two-bedroom annexe — comfortable space for a couple, or a parent plus visiting family, with room for a proper living area.
Three-plus bedroom annexe — approaching the upper end of what's achievable, suited to larger families or those wanting significant extra space.
What limits your actual build size, beyond the legal maximum?
The 136m² figure is a ceiling, not a target — plenty of annexes are built well below that size, because it's exactly what the family needs. What actually determines your realistic size includes:
Garden size and shape. The physical space available on your plot is often the biggest practical constraint, well before you approach the legal maximum.
Access. Getting materials and equipment to the build site, and ensuring reasonable access to the finished annexe, can influence how large and where a build is feasible.
Existing services. The distance and complexity of connecting power, water and sewage to the annexe location can factor into what's practical.
Distance from the main house. Some councils and building regulations have expectations around separation distances between the annexe and the main property, which can influence layout and positioning.
Neighbouring properties. While full planning permission usually isn't required, being considerate of neighbouring boundaries and privacy is still good practice — and can affect where and how large a build makes sense.
Do you need to use the full 136m²?
Not at all — and in our experience, most families don't. The right size is the one that suits the people actually going to live there, your garden, and your budget. Building larger than necessary adds cost and construction time without necessarily adding value, particularly if the annexe is intended for a single person or couple.
What if you need more space than the Caravan Act allows?
If your family's needs genuinely require a larger structure than the Caravan Act permits, that doesn't mean the project is off the table — it just means the route changes to full planning permission instead of a Certificate of Lawful Development. This is a longer and more involved process, but it remains a legitimate option for families who need more space than the Act allows. We can talk through both routes and what's realistic for your specific circumstances during your consultation.
Getting the size right from the start
Because size affects planning route, cost, build time and day-to-day livability, it's worth getting real clarity on this early — not adjusting it partway through the design process. Our site survey and initial consultation is specifically designed to help you understand what's genuinely achievable on your plot, and what size actually makes sense for your family's needs, before any formal application goes in.
The bottom line
Granny annexes built under the Caravan Act can go up to 136m², giving real flexibility from a compact studio through to a full multi-bedroom home — but the right size for your project depends on your garden, your family's needs, and your budget, not just the legal maximum. A proper site survey is the only reliable way to establish what's genuinely achievable for your specific property.
Not sure what size granny annexe or garden annexe is right for your garden? Get in touch with Mobile Annexe Ltd for a free consultation and site survey. We build across Essex, Hertfordshire and the rest of the UK, and we'll help you work out exactly what's achievable — and what genuinely suits your family.
📞 07770 365734 ✉️ enquiries@mobileannexe.com
Comments