The start of spring is often a golden moment for most construction and renovation projects, but it can be particularly important if you want to install a bespoke annexe building.

Whether it is to create a place of peace and solitude near your home, to provide an additional revenue stream or to allow you to remain close to older members of your family whilst also ensuring they can live freely and independently.

There are as many unique uses for garden annexes as there are ways to build them, and we offer a bespoke design service to ensure you get the exact building you need.

However, one of the biggest questions we are asked is about size; how big can a garden annexe building be? How many bedrooms can it have? When do you need planning permission? And what might cause it not to be granted?

The answers are complex, to say the least and might depend on where you live. However, we will attempt to demystify and explain what you can and cannot do.

Do You Need Planning Permission For A Garden Annexe?

Ultimately, whether you need planning permission for your annexe building depends on how big it is, where you live, your local planning authority, and what you plan to use it for.

In general, garden annexes that are not intended to be lived in (such as a home gym, office, workshop, garden room, dining areas and so on) are classified as “outbuildings” under UK planning rules as long as they meet certain conditions.

These rules vary by planning authority, but the general conditions are as follows:

  • They cannot be taller than 2.5 metres, which means, by definition, no two-floor buildings are allowed.
  • They cannot have a raised platform, so no balcony.
  • They cannot, in combination with other extensions, cover half of the land outside of the original house.
  • They cannot be constructed without planning permission in “designated land” areas, which include conservation areas and near heritage sites.
  • They cannot be constructed within the grounds of listed buildings without planning permission and listed building consent.
  • An Article 4 Direction or alternative planning condition can affect permitted development rights more broadly.

As with any planning rules, it is always best to contact your local planning authority first with your initial project ideas, and they will be able to tell you if you can proceed, what you need to be mindful of and whether you need planning permission.

Asking for forgiveness instead of permission can be especially costly.

Do You Need Planning Permission For A Granny Annexe?

Granny annexes, and other self-contained annexe buildings, are much more difficult to determine the planning permission they need, to the point that some local planning authorities warn that they cannot provide advice over the phone, as they often can for other permitted development projects.

In general, however, self-contained living accommodation is not allowed, with the definition of “self-contained” usually meaning that cooking facilities, a bathroom and a bedroom are included so someone could live by themselves in it.

There are some complicated exceptions; converting an existing shed, garage or other outbuilding into accommodation can be allowed as long as it is not considered to be self-contained, and some other types of annexe are allowed if they qualify under other permitted development rules.

In general, it is best to work under the assumption that you will require planning permission, and a major part of our design process involves planning consultancy to ensure that the building is lawful.

How Likely Is It that A Garden Annexe Will Get Planning Permission?

Whilst the specifics will vary by planning authority, a garden annexe will generally be approved as long as it fits within your land, is of a high quality and does not cause any problems to neighbours.

Most planning applications are examined on a case by case basis, but alongside these core principles, there are also precedents and expectations, such as:

  • It should be located in the garden.
  • It should have a “functional connection” to the main house, which means that the person staying there should have some kind of familial or professional connection.
  • It should be used in conjunction with the house.
  • It should be owned by the same people who own the main house.
  • It should be designed to be converted into an integral part of the main house.
  • It cannot have a wall or other boundary surrounding it.

When Is A Garden Annexe Building Too Large?

A garden annexe cannot be larger than the main house it is located in, but it also should not be larger than half the size of the main house. Plans for some larger granny annexes have struggled if they are larger.