Last Updated: January 2026

Building a granny annexe is one of the most practical ways to support a family member while keeping everyone close. But when it comes to granny annexe planning permission, these buildings sit in a more complex category than your standard garden rooms or summer houses.

Whether you need permission depends on how the annexe will be used, how it’s connected to the main house, and whether it’s considered part of the existing dwelling or a separate unit.

In some cases, it’s possible to build without planning permission, but the conditions are strict and interpretation varies by council. This means that you always need to research and meet your local requirements, which might differ from what we have outlined in this article. 

Another important thing to remember is that buildings that are used to accommodate a person overnight may need building regulations approval and you may also need a certificate of lawfulness.

Let’s dive into when planning permission is required for a granny annexe, the different routes available, and what to check before you build.

Building your granny annexe: When do you need permission?

As a very short summary, you can either build your new granny annexe a) without planning permission as permitted development or as a ‘caravan’, b) with planning permission or c) by changing the use of an existing building (which may also be possible without planning permission). Let’s have a closer look at the different concepts and rules you should know about.

Permitted development

As an owner of a property, you have permitted development rights. This means you have the right to build certain buildings on your land without planning permission. 

You’re allowed to build things like garden rooms, sheds or garages without planning permission under class E of permitted development. This is as long as they meet certain requirements – usually to do with the size and use of the building.

It’s good to know that permitted development under class-E does not cover: ¹

• Listed buildings, World Heritage Sites, National Parks, areas of outstanding natural beauty and the Broads.
• Buildings that have more than one story
• Buildings that are higher than:
  • 4 m (about 13.1 ft) in the case of a building with a dual-pitched (apex) roof
  • 2.5 m (about 8.2 ft) if the building is located within 2 metres of the boundary of the curtilage of the main house
  • 3 m (about 9.8 ft) in all other cases
• Buildings where the height of the eaves (the lowest point of a roof) would exceed 2.5 metres
• When a planned veranda, balcony or raised platform exceeds the height of 0.3 m (about 0.9 ft)
• When the total area of the covered land by both new or old buildings (not including the main house) would exceed 50% of the land (not including the land covered by the main house).
• When the total area of ground covered by buildings located more than 20 metres from any wall of the main dwelling would exceed 10 square metres (about 32.8 ft).

Permitted development limits (UK)

Requirement	Permitted development limit
Maximum floor area	Up to 30m² (non-sleeping use)
Number of storeys	Single storey only
Height within 2m of boundary	2.5m total height
Height beyond 2m of boundary	3m (flat/pent roof) / 4m (apex roof)
Eaves height	Max 2.5m
Garden coverage	Max 50% of land around original house
Balconies / raised platforms	Not permitted over 0.3m
Use as living accommodation	Not permitted

If you’re exploring a granny annexe, starting with a well-designed garden room can offer flexibility while you confirm planning requirements.

Incidental and ancillary use explained

To understand what you’re allowed to build with or without planning permission, we need to look at the terms ‘ancillary’ and ‘incidental’.

You are allowed to add buildings without planning permission when they are incidental to the enjoyment of the main dwelling. Incidental use is considered something you wouldn’t typically have inside your home like storage, a workshop or a swimming pool.

If a building is considered ancillary in a planning context, it will often need planning permission. Ancillary buildings are considered an extension of the main house and include things that can be done in the main house, like cooking or sleeping.

While the general approach is the same, every council will have its own rules in place. Make sure you understand the relevant requirements in your area. You can find and get in touch with your local council through the National Planning Portal. The Planning Portal should be your first stop for all planning-related information and for submitting applications.

Your council will look at many things to try to conclude the purpose of a building and whether it meets necessary requirements or not. The reality is that these interpretations can come down to local policies and individual opinions. An annexe that can be built with or without planning permission in one area may not be approved in another under the same conditions.

In practice, councils look closely at whether an annexe could function independently. Features such as a full kitchen, separate access, individual utility meters or postal addresses can all influence how a building is classified. Even when a structure appears modest in size, these details can shift it from incidental use into ancillary or self-contained accommodation, which usually requires planning permission.

Caravan Sites Act 

It’s possible to build a granny annexe without planning permission by using the Caravan Sites Act of 1968. ² This is because under certain conditions, even a granny annexe may be considered a ‘caravan’. And no, it doesn’t need to be a mobile home. It just needs to meet some requirements.

The Caravan Sites Act defines ‘Caravan’ as follows:

• It is composed of not more than two sections separately constructed and designed to be assembled on-site by means of bolts, clamps or other devices; and
• It is, when assembled, physically capable of being moved by road from one place to another (whether by being towed or by being transported on a motor vehicle or trailer)

Granny annexes that are designed in a way where they can theoretically be taken apart and transported in two pieces meet these requirements. Even when you have a connection to plumbing and electricity, the annexe can be built under the Caravan Act.

The garden annexe also needs to meet the following size requirements:

• Less than 20 m or 65.616 ft in length (exclusive of any drawbar)
• Less than 6.8 m or 22.309 ft in width
• Less than 3.05 m or 10.006 ft in height (internal floor lowest point to internal ceiling highest point)

While the Caravan Sites Act can provide a lawful route in specific cases, councils often scrutinise these applications closely. Interpretation varies, and what qualifies in one area may be challenged in another. Professional advice is strongly recommended before relying on this route, particularly where services, foundations or long-term occupation are involved.

Converting a garage into a granny annexe

If you’re looking to convert an existing garden room or a garage into a granny annexe, it’s entirely possible, but there are a few things to know about. We’ll look at the differences between converting an attached building versus an outbuilding, which is separate from the main home.

Converting a detached garage or garden room

When you have an existing garden room or summer house and you’re already using it (provided the building has a Lawful Development Certificate) or it is an incidental building, you can apply for a change of use once you convert it to an annexe. 

You would also request a change of use for any other outdoor building (like a detached garage) that you’d be converting into the granny annexe.

However, you may still need planning permission, depending on your situation – make sure to check your local requirements. But what do you do when the garage is a part of the main house?

Converting an attached garage to a granny annexe

Here, you may be able to carry out the works using your permitted development rights, which is a great advantage. Typically, this means that if you don’t make significant changes in the exterior or the footprint of the building, you don’t need permission. Again, make sure to check the local requirements regarding this.

You will also need to check if there are any planning conditions attached to your home. These are restrictions, like needing to maintain the garage as a place to park the car. If you do have such a restriction, you can apply to have the restriction removed.

Lawful Development Certificate and building regulations

It’s important to remember that even without planning permission, you will still need to apply for a Lawful Development Certificate (LDC). ³ For example, if your granny annexe meets the requirements for a caravan, you would still apply for an LDC. This is to essentially show that the building doesn’t need planning permission and is lawful.

When a building can be used to stay in overnight, it is more likely that planning permission may be required. Building regulations approval, however, is always required in this case. This is because a garden building where people can sleep has stricter safety requirements.

One way to get around applying for building regulations approval yourself is to have the work carried out by a tradesperson registered with the competent person scheme. ⁴

How to build a granny annexe without planning permission

To recap, here are the ways you can build your granny annexe without getting planning permission:

1. Build a garden room or gym under your permitted development rights to later convert to an annexe. Or, transform your existing summer house or garage. Then apply for a change of use and convert it to a self-contained annexe.
2. Convert an attached garage or another part of the main house into a granny annexe.
3. Take advantage of the Caravan Sites Act of 1968. It’s possible to build an annexe without planning permission under this law, as long as it complies with a few rules.

You’ll also need to make sure that the works are carried out safely by professionals according to building regulations and apply for an LDC.

What happens if I build a granny annexe without planning permission?

If you’ve done the work without getting planning permission when you needed one, you may be served an enforcement notice. This notice will order you to undo all the work that has been carried out and you may also be fined.

This is why it’s very important to be absolutely certain that you don’t need planning permission before building your granny annexe. Even when you have no issues in the short term, the situation can become a real headache if you ever need to sell your property. It’s also worth noting that enforcement notices can be appealed. ⁵

Tips for a successful planning permission application

Size is the most frequent reason for an annexe to be refused permission. Most refusals will come down to this factor, so keep the building on the smaller side. This shows that the annexe is subordinate to the main home. If possible, try to have just one bedroom and one open-plan living room-kitchen.

One of the main concerns that councils have is that the outbuilding will become its own separate dwelling when it has its own access or parking space, for example. The more you’re able to demonstrate that an ancillary granny annexe can’t be rented out or sold separately in the future, the likelier it is that you’ll get your planning permission approved.

To really emphasise that the annexe is a part of the main house, build the granny annexe as close to it as possible. Try to find other ways to show it’s not an independent building, for example, you could use the same water and electric connection that is not metered separately.

Another important thing that is taken into account is how the building will impact the neighbours. This is why you may want to choose a flat roof instead of an apex roof, if there are several neighbours whose properties may be overlooking yours.

Choosing an annexe-style building that complements your main home can make the planning process smoother and reduce objections.

Last thoughts

Building a granny annexe doesn’t need to be complicated, especially if you choose a high-quality prefabricated annexe like the ones made by Summerhouse24. Manufactured to the highest standard, these beautiful wooden cabins make for wonderful annexes that last you for a lifetime. Do you have any questions or need help choosing an annexe? Don’t hesitate to get in touch.

FAQ’s

Sources used:

1. https://www.planningportal.co.uk/ 
2. https://www.legislation.gov.uk/ukpga/1968/52 
3. https://www.planningportal.co.uk/permission/responsibilities/planning-permission/lawful-development-certificates 
4. https://www.gov.uk/building-regulations-approval/when-you-dont-need-approval 
5. https://www.gov.uk/planning-permission-england-wales