Do You Need Planning Permission for a Granny Annexe?

If you're thinking about adding a granny annexe or garden annexe to your garden, this is usually the first question we get asked — and it's the one that puts most people off before they've even looked into it properly. The good news is that in most cases, the answer is no.

Here at Mobile Annexe Ltd, we build granny annexes and garden annexes across Essex and Hertfordshire, as well as the rest of the UK, and we take homeowners through exactly this process every week. Here's how it actually works.

The short answer

Most granny annexes and garden annexes don't need full planning permission. Instead, they're built to fall under the Caravan Sites and Control of Development Act 1960 (usually just called the Caravan Act), which classes them as a type of mobile home. As long as the annexe meets the size and design rules set out under the Act, you only need a Certificate of Lawful Development from your local council — not full planning consent.

That's a much simpler, faster and cheaper process, and it's the route we use for the vast majority of our builds.

Why the Caravan Act matters

The 1968 Caravan Act sets out clear rules on what counts as a "caravan" for planning purposes — and it's broader than most people expect. Granny annexes, garden rooms, mobile homes and other transportable buildings can all be classed and treated the same way under the Act, provided they meet certain conditions around size, construction method and how they're used.

Because your annexe is legally treated as a caravan rather than a permanent structure, it sits outside the normal planning permission system. That's the loophole (a perfectly legal one) that makes granny annexes such an attractive option compared to a traditional extension.

What you actually need instead: a Certificate of Lawful Development

Rather than planning permission, you'll need a Certificate of Lawful Development (CLD). This is a formal document from your council confirming that what you're building is lawful and doesn't require planning consent.

It's not automatic — you do have to apply for it, and the council will check your plans against the relevant rules. But it's a far lighter-touch process than a planning application, and there's no lengthy public consultation or committee decision involved.

We handle this application on your behalf as part of our service, and we currently have a 100% success rate getting Certificates of Lawful Development approved for our clients.

When it's not classed as "incidental" use

There's one condition worth understanding clearly: your annexe needs to be used in a way that's incidental to the main house — for example, housing an elderly parent or an adult child as an extension of the family home.

If the annexe is instead treated as a fully separate, independent household — for instance, if it's let out to unrelated tenants, or the "family" connection stops applying — councils can decide that a material change of use has taken place. At that point, the rules are different, and you may need to apply for full planning permission after all.

In practice, this rarely causes issues for genuine granny annexes built for family use. But it's worth being upfront with your builder (and your council) about exactly how the space will be used, so everything's set up correctly from day one.

Size and design restrictions

Because your annexe is classed under the Caravan Act, there are limits on size and construction that keep it within the rules. As a general guide, we can build up to 136m², giving scope for anything from a compact studio to a full one-to-five bedroom annexe — but the exact specification needs to be checked against the current rules for your plot and council area, since local interpretation can vary slightly.

This is exactly why it's worth using a builder who's dealt with the process many times before, rather than assuming every council will apply the rules identically. We work with councils across Essex and Hertfordshire on a regular basis, from Uttlesford and Stansted through to East Herts and beyond, so we know how each local planning department tends to approach granny annexe and garden annexe applications.

What about full planning permission?

Some homeowners do choose — or need — to go down the full planning permission route instead, particularly if:

• The annexe will be larger than the Caravan Act rules allow

• The property has specific restrictions attached

If that's the case for your project, it doesn't mean you can't build a granny annexe — it just means a different (and longer) application process. We can talk you through both options and the associated build and cost timelines during your free consultation.

Council tax and other practical implications

It's worth knowing that once your annexe is occupied, as the resident you'll typically be personally liable for council tax on it — usually at a lower band than the main house, since it's a smaller, self-contained unit. This is separate from the planning question, but it's one of those practical details that catches people out later in the process, so it's worth factoring into your budgeting from the start.

The bottom line

For the majority of granny annexe and garden annexe projects, you won't need to go through full planning permission — a Certificate of Lawful Development under the Caravan Act is the route most families take, and it's considerably faster and simpler. But every plot, property and council is a little different, which is why a proper site survey and planning conversation should always be step one, not an afterthought.

Thinking about adding a granny annexe or garden annexe to your garden? As specialists based in Stansted, on the Essex and Hertfordshire border, Mobile Annexe Ltd offers a free consultation to homeowners across both counties, as well as the rest of the UK — we'll walk you through exactly what's needed for your specific property, and handle the Certificate of Lawful Development application for you from start to finish.

📞 07770 365734 ✉️ enquiries@mobileannexe.com