The planning unit and how it is identified

Before you can say whether a change of use is material, you have to decide what area you are judging it against. That area is the planning unit, and planning unit identification is the first step in almost every change of use question, every enforcement notice and every lawful development certificate application. Get it wrong and the rest of the analysis collapses, because a change that looks material across a small area can be trivial across a larger one.

The starting point is the unit of occupation. From there the courts have built a short set of categories that tell you when to stay with that unit and when to subdivide it.

Why the unit has to be fixed first

Section 55(1) of the Town and Country Planning Act 1990 defines development to include a material change in the use of any buildings or other land. The word "material" does the work, and materiality is relative. A hot food counter occupying 8 m² inside a 600 m² supermarket is a detail of the retail operation. The same counter in a 30 m² shop is the operation.

So the question is never "is this change material" in the abstract. It is "is this change material to the planning unit". Case officers who skip that step tend to allege the wrong unit in a notice, and notices fail on that basis.

The unit also sets the baseline for lawfulness. When you apply under section 191 for a certificate of lawful existing use, you are asserting a use of a defined area. Describe the area too widely and you ask the council to certify something you cannot prove. Describe it too narrowly and the certificate is worth little.

The three categories the courts use

In Burdle v Secretary of State for the Environment the High Court set out the approach that is still applied. The burdle categories planning unit analysis runs as follows.

Where all the activities on a unit of occupation form part of a single main purpose, the unit of occupation is the planning unit, even though there are incidental or ancillary activities within it. A joinery workshop with an office, a staff kitchen and a small trade counter is one unit in a single main purpose use.

Where the occupier carries on a variety of activities and none of them can fairly be called incidental or ancillary to any other, the whole unit of occupation is again the planning unit. This is the composite or mixed use case. The unit has a mixed use, and that mixed use is what has to be described.

Where two or more physically separate and distinct areas within one unit of occupation are used for substantially different and unrelated purposes, each area is a separate planning unit. A farmyard where one building has been let for vehicle repairs behind its own gate is the common example.

Occupation and physical separation as tests

Occupation is the usual starting point rather than ownership or title. A landlord who owns a parade of six shops does not have one planning unit covering the parade, because each shop is separately occupied. Conversely a single occupier spread across two adjoining buildings with an internal opening between them may hold one unit.

Physical separation matters most in the third category. Look for the things that make an area function on its own: a separate entrance, its own yard, a fence or wall, separate services, separate parking, a distinct address. One of those on its own rarely decides it. Several together usually do.

The test is not purely visual. An area can be physically distinct and still be part of a single main purpose, as a detached store serving a factory is. What you are looking for is separation plus a different and unrelated purpose.

A short worked check

This is an illustrative example. A two-storey building has a florist at ground floor and a tattoo studio above, each with its own street door, separate meters, no internal connection and unrelated occupiers. Two planning units. Change the facts so that the florist uses the first floor for stock and staff, and there is one unit in retail use with ancillary storage above.

Effect on whether a change is material

Once the unit is fixed, materiality is judged by reference to the character of the use of that whole unit. Intensification can be material, but only where it changes the character of the use rather than simply increasing it. The threshold question often ends up being one of degree, which is why the de minimis principle in planning judgments sits so close to this topic.

Ancillary activity can grow until it becomes a main purpose in its own right. A farm shop selling the holding's own produce is ancillary. The same shop stocked mainly from wholesalers is retail, and the unit's use has changed. Nothing about the building needs to alter for that to happen.

Subdividing a unit and the consequences

Subdivision is itself capable of being development. Dividing a single dwellinghouse into two flats is expressly a material change of use under section 55(3)(a). Outside the dwellinghouse context, subdivision is assessed on ordinary materiality principles, and the creation of two units from one often changes the character of what is happening on the land.

The practical consequence is loss of flexibility. Two smaller units each carry their own planning history, their own permitted development allowances and their own ancillary tolerances. Movement of activity between them is no longer internal. Clients who split a yard to let part of it are frequently surprised that they have also split the fallback position they were relying on.

Reversing a subdivision is not automatic either. Merging two units back into one can be a further material change requiring permission.

Enforcement and the unit alleged

An enforcement notice has to specify the land to which it relates and the breach alleged. If the council alleges an unauthorised use across an area that is in truth two planning units, the notice may be defective, and that is a ground of appeal in its own right. The reverse error, alleging too small an area, can mean the notice fails to describe a material change at all.

Where you are advising on a notice, pull the title plan, the rating list entries, the tenancy documents and any historic aerial imagery before arguing about merits. The unit argument is often stronger than the planning case. It is also the argument most often left out of a hurried appeal statement, in the same way that the detail of what was actually approved gets lost between consent and the discharge of conditions.

Identifying the unit on a mixed site

On a large mixed site, work from occupation outwards. List every occupier, then every area in separate occupation, then test each area against the three categories. Only after that should you form a view on the character of the use of each unit.

Record the evidence as you go: a plan with each unit edged and numbered, the occupier and the dates, the access arrangements, the services, the rateable units. That evidence pack is what a case officer or an inspector will ask for, and assembling it afterwards is much harder.

Where an outline consent is in play the boundaries may already be settled by the approved plans, which narrows the argument considerably, and the scope of what remains open at reserved matters stage follows the same logic of asking what has already been determined.

A draft Planning Statement can set out the unit, the categories and the evidence in a defensible structure, and PlanScope will flag the site facts it does not hold rather than filling them in. Enter your postcode on PlanScope to see which reports the site is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that a lawful development certificate is the only formal way to confirm that an existing or proposed use is lawful.

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