What Is the Definition of a Planning Unit?

If you have submitted a planning application, you may have heard planners talk about the “planning unit”. It sounds technical, but the idea is actually quite simple — and very important.

Understanding what the planning unit is can be the difference between something being lawful without planning permission, or needing permission (and possibly enforcement action).

This article explains what a planning unit is, why it matters, and where things can become legally uncertain.

In Simple Terms: What Is a Planning Unit?

A planning unit is the area of land or buildings that planning law looks at when deciding how a site is being used.

It is the “unit” against which the planning system asks questions like:

• What is the main use of this land or building?

• Has that use changed?

• If it has changed, is that change material (i.e. does it need planning permission)?

The key point is this:

Planning law does not always look at individual rooms, sheds, or bits of land in isolation. It usually looks at the site as a whole — the planning unit.

Why the Planning Unit Matters

The concept matters most when deciding whether there has been a material change of use (MCU).

Before anyone can answer “has the use changed?”, they must first answer:

What is the correct planning unit?

Get that wrong, and the conclusion on whether planning permission is needed may also be wrong.

This approach has been confirmed repeatedly by the courts, most notably in Burdle & Williams v Secretary of State [1972], the leading case on planning units.

The General Rule: The Unit of Occupation

As a starting point, the planning unit is usually the unit of occupation.

That means:

• What is occupied together

• By the same person or business

• For a broadly consistent purpose

For example:

• A house, its garden, and its garage are normally one planning unit

• A factory complex is often one planning unit

• A single shop unit is usually one planning unit

This was reinforced in cases such as G Percy Trentham Ltd v MHLG [1966], where storage inside a farm building was treated as part of the farm’s planning unit, not a separate storage use.

When Can a Smaller Planning Unit Exist?

Sometimes, a site that looks like one property can actually contain more than one planning unit.

The courts have said this depends on fact and degree, focusing on two key ideas:

1. Physical separation

Is the area clearly separate on the ground?

2. Functional separation

Is it used independently, for a different and unrelated purpose?

Both are important.

As the courts put it in Burdle & Williams, you generally need both physical and functional separation before you can identify a smaller planning unit.

Three Common Planning Unit Scenarios

The courts have identified three broad situations.

1. One Planning Unit with Incidental Uses

This is the most common situation.

There is:

• One main (primary) use, and

• Other activities that are incidental or ancillary

Example: A house with a home office, hobby workshop, or occasional storage. As long as those activities remain secondary, the planning unit stays residential.

This principle was confirmed in Vickers-Armstrong Ltd v CLB [1957], where changes between ancillary uses in a large factory did not amount to a material change of use.

2. One Planning Unit in Mixed Use

Sometimes, no single use is clearly “secondary”.

Example:

• A house where a substantial car-repair business operates from the garage

• A building used equally as a café and a shop

In these cases, the planning unit may be in mixed use.

The planning unit is still one unit — but its lawful use now consists of more than one activity.

3. Separate Planning Units on the Same Site

Where parts of a site are:

• Physically separate, and

• Used for different and unrelated purposes,

they may each be their own planning unit.

Example: A garage that is:

• Let to a separate business

• Operated independently from the house

• Functionally unconnected to the dwelling

In that situation, the house and garage are likely to be different planning units.

This approach has been supported in cases such as Johnston & Johnston v SSE [1974], where individual garages were treated as separate planning units based on occupation.

Situations Where the Planning Unit Is Often Disputed

Some scenarios regularly cause difficulty.

Home Businesses

Working from home does not automatically create a new planning unit.
But if:

• The business activity becomes dominant, or

• Parts of the property are functionally separated,

the planning unit — and its lawful use — may change.

Large Sites with Multiple Activities

Farms, industrial estates, and leisure sites often host multiple uses.

For example:

• Markets or boot fairs on farmland are often treated as part of the farm planning unit, not individual fields

• Coordinated activities across different land parcels can still form one planning unit

This was upheld where boot fairs operated across adjacent land in different ownerships.

Can Planning Permission Be “Lost” by Creating a New Planning Unit?

Yes.

In Stone & Stone v SSCLG [2014], the court confirmed that:

• Creating a new planning unit can extinguish a previously authorised use

• Whether this has happened is a question of fact and degree

This is particularly important where land is subdivided or re-occupied in new ways.

However. local planning authorities cannot artificially divide a planning unit just to make enforcement more restrictive. This principle comes from De Mulder v SSE [1973].

However:

• Enforcement notices do not always have to cover the whole planning unit

• Authorities can enforce against part of a site if justified

The key test is whether doing so causes injustice.

Key Takeaways

• A planning unit is the area of land or buildings assessed as a whole for planning purposes

• It is usually the unit of occupation

• Smaller planning units only exist where there is real physical and functional separation

• Many disputes come down to fact and degree, not rigid rules

• Getting the planning unit wrong can lead to incorrect conclusions about lawful use or enforcement

Why This Matters in Practice

If you are:

• Changing how part of a property is used

• Letting out buildings separately

• Introducing new commercial or leisure activities

• Facing planning enforcement

…the definition of the planning unit is often the starting point — and sometimes the deciding factor.

If you’re unsure, professional planning advice early on can prevent costly mistakes later.

About Me

Andrew Ransome is the planning director at ADP and is a chartered member of the RTPI, with over 22 years of town planning experience.

Andrew has extensive experience offering strategic planning solutions to challenging projects in both rural and urban settings. Follow him on Linkedin.

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