Planning unit: what is it?

A planning unit is the area of land whose use is being assessed for planning purposes. Under the Burdle approach it is usually the unit of occupation, unless a smaller area can be identified which is physically separate and distinct and is occupied for substantially different and unrelated purposes. Boundaries, occupation and the way the land functions are important evidence. Common ownership may be relevant background, but ownership on its own does not decide the planning unit.

On this page we explain what a planning unit is, and the difference between ancillary and incidental uses – two terms that get used interchangeably but mean genuinely different things in planning. A garden shed or detached garage used for normal domestic purposes will often be incidental to the enjoyment of the dwellinghouse. A granny annexe may remain ancillary to the same dwellinghouse, but that is not automatic: its occupation and functional relationship with the main house determine whether it remains within the same planning unit or has become a separate dwelling. The distinction can affect whether a use is lawful, whether development has occurred and whether planning permission is needed.

The Planning Unit

In Burdle v Secretary of State for the Environment [1972] 3 All ER 240, Bridge J held that the planning unit is usually the unit of occupation, unless a smaller area can be identified which, as a matter of fact and degree, is physically separate and distinct, and occupied for different and unrelated purposes. The concept of physical and functional separation is key, and Justice Bridge suggested 3 broad categories of distinction:

• Where it is possible to recognise a single main purpose of the occupier’s use of his land to which activities are incidental or ancillary, the whole unit of occupation should be considered the planning unit

• Even though the occupier carries on a variety of activities, and it is not possible to say that one is incidental or ancillary to another, the entire unit of occupation should be considered the planning unit, in mixed use

• Where there are 2 or more physically separate and distinct uses, occupied as a single unit but for substantially different and unrelated purposes, each area used for a different main purpose (together with its incidental and ancillary activities) ought to be considered a separate planning unit

Many planning units have an identifiable primary use, but not all do. Under the Burdle approach a planning unit can also have a genuine composite or mixed use where no one activity is ancillary to another. In the case of a dwelling house, the primary use will be the main dwellinghouse and any associated garden. It can also include one or more ancillary or incidental uses that do not alter the primary use of the land, providing they are closely linked and subservient to it. On a farm the primary use would be agriculture.

If a building or use is no longer properly part of the same planning unit, or a material change of use has occurred without permission, enforcement may become an issue. But the move from incidental to ancillary use does not automatically amount to a material change of use. The real question is whether the overall planning unit and its character of use have materially changed. Previous lawful uses and any planning conditions can also matter.

A planning unit and a new planning unit are not the same thing

These two phrases sound alike and get muddled constantly, including by professionals. They do completely different jobs.

The planning unit above is a measuring tool. You use it to work out which piece of land to look at when deciding whether the use of that land has changed enough to need permission.

A new planning unit, often called a new chapter in the planning history, asks something else entirely: whether putting up a new building, or carrying out a permission you were granted, has wiped out a use right you already had, including any right you had to go back to a previous lawful use. The test there is whether the new development sits so awkwardly with the old use that the two cannot both survive.

The Court of Appeal set this out in Titchfield Festival Theatre Ltd v Secretary of State for Housing, Communities and Local Government [2026] EWCA Civ 368, and suggested sticking to a new chapter in the planning history for the second one, precisely so the two stop being confused.

Incidental Uses

For example a dwelling house with a detached garage would be an incidental use. One dictionary definition is: “Of a minor, casual, or subordinate nature”. 

In the case of land, this is likely to be under say a farm ownership and the land is all used for agricultural purposes. If the farm creates buildings which are used for storage of farm equipment, these would be incidental uses and not separate planning units. 

An incidental use is subordinate in land-use terms to the primary use and exists for the enjoyment of, or in connection with, that primary use. In the domestic context, a home gym or hobby room used by the occupants can be incidental to the enjoyment of the dwellinghouse. On a farm, a tractor shed serving the agricultural enterprise is ordinarily part of the agricultural use rather than an example of the domestic planning concept of incidental use. In a residential setting, an incidental use can include facilities that also appear in living accommodation, such as a shower serving a gym or swimming pool. The label attached to a room is less important than its actual relationship to the primary use.

Incidental domestic uses can include a garage, storage, home office, gym, art studio, pool pump room, hobby room, cinema room, shed, pergola or decking, depending on the facts and the way the space is used. 

It is possible that any of these incidental uses, could be considered not to be incidental due to size and the fact that they are duplicating a use found within the main building. You can see how this can become very confusing, very easily and why some local authorities or planning officers or enforcement officers may differ in their opinion from others. Often we might suggest a certificate of lawfulness to confirm that the building and use is incidental. Planning Geek can assist you with this. Please request a fee proposal if you would like us to assist.

An incidental use can easily become an ancillary use.

An incidental use remains part of the planning unit.

Ancillary Uses

A detached building used as a granny annexe may remain ancillary to the same C3 dwellinghouse and within the same planning unit. There is no legal requirement for it to exist first as an incidental use and only later become ancillary. Whether it remains part of the same planning unit is a matter of fact and degree, including how the accommodation is occupied and its functional relationship with the main house.

Ancillary residential accommodation can duplicate facilities found in the main house. A granny annexe may contain a bedroom, bathroom, kitchen and sitting room and still remain part of the same C3 planning unit. Self-containment is relevant, but it is not decisive on its own. The question is whether, as a matter of fact and degree, the accommodation has become a genuinely separate planning unit with an independent residential use.

Staff accommodation in a separate building may also be ancillary where the occupation remains functionally tied to the main use. Again, that is a matter of fact and degree rather than an automatic rule.

Ancillary residential accommodation may remain within the same planning unit even where it contains substantial domestic facilities. Dependence on facilities or services in the main dwelling can be useful evidence, but self-containment is not decisive by itself. The overall occupation and functional relationship are what matter.

A lawful building can be put to an ancillary use without necessarily creating a material change of use. Planning permission is required for the use only if the change amounts to development, for example because a materially different or independent use has arisen, or where a planning condition or other control prevents it.

An ancillary use remains part of the planning unit.

New Planning Units

Once land or buildings are divided off to provide two or more physically separate and distinct uses for differing purposes, this may well be considered a separate planning unit. Note that this would usually involve a change of use of the land or buildings and would therefore require planning permission unless it was a permitted development right. 

Examples of creating a separate planning unit, might include division of a commercial unit, or selling off land to a new farmer to operate their own separate business, or getting planning to build a new house on part of the garden. Note that the same ownership does not prevent two or more planning units – it is what happens within land and buildings that count.

So a farmer, who sets up an equestrian school (subject to planning) may well have created a separate planning unit as this would be considered a substantially different and unrelated purpose.

Other Common Projects

Planning Unit Page Updated: 2nd September 2026