The planning unit: how councils decide what they assess

Before anyone can say whether there has been a material change of use, they have to say what land they are looking at. That is the planning unit test, and it decides cases. A first floor storage room that is part of a shop is not a separate use. The same room, let separately with its own access and its own occupier, probably is. Nothing in the Use Classes Order helps until the unit is fixed.

The starting point comes from Burdle v Secretary of State for the Environment (1972). The correct planning unit is normally the unit of occupation, unless and until some smaller unit can be recognised as the site of activities that amount in substance to a separate use, both physically and functionally. Read that sentence twice. Both limbs have to be satisfied, and applicants regularly argue physical separation while ignoring the functional half.

Why the unit comes before the use class

Section 55 of the Town and Country Planning Act 1990 makes a material change in the use of any buildings or other land into development. "Any buildings or other land" is not self defining, so the question of what is being assessed has to be answered first.

Two practical consequences follow immediately.

An activity that looks like a different use may be ancillary within a larger unit, and so not a change of use at all. A farm shop selling the holding's own produce sits inside an agricultural unit. A farm shop selling mostly bought in goods, with its own car park and staff, has probably become its own retail unit.

And an intensification argument depends entirely on the boundary drawn. Spreading the same activity across a larger unit dilutes it. Concentrating it in a smaller one makes the change look material. Our note on intensification and when more of the same becomes a new use covers that side of the analysis, and the earlier piece on identifying the planning unit sets out the basic concept if you want the shorter version.

The three situations in the case law

Burdle identified three situations, and almost every argument about the planning unit can be sorted into one of them.

First, where the occupier has a single main purpose and the other activities are incidental or ancillary to it. The whole unit of occupation is the planning unit. A garage with a workshop, a small parts counter and a valeting bay is one unit with one use.

Second, where the occupier carries on a variety of activities and none can fairly be called incidental to another. This is a composite use, and again the whole unit of occupation is the planning unit. The use is described as the mixture, not as one of its parts.

Third, where within a single unit of occupation there are two or more physically separate and distinct areas occupied for substantially different and unrelated purposes. Here each area is its own planning unit.

That third situation is the narrow one. Physical separation on its own is not enough, which is why a mezzanine, a partition or a separate room rarely creates a new unit by itself. What is needed is an area that functions independently: its own access where possible, its own servicing, its own occupier or a distinct operation under the same occupier that does not serve the main activity.

Occupation, control and physical separation

Occupation is the organising idea, and it is about control rather than ownership. A landlord owning a parade of six shops has six planning units, because each is separately occupied. A single tenant taking two adjoining shops, knocking through and running one business has created one unit from two.

Questions worth asking about any site where the unit is contested:

• Who has control of each area, and under what arrangement?
• Is there separate access from the highway or from a common area?
• Are the areas separately serviced, metered, and separately rated?
• Could one area operate if the other closed tomorrow?
• Does the activity in one area exist to serve the other?

The last question is the ancillary test, and it is the one that decides most disputes. Ancillary means subordinate and serving. It does not mean smaller. A large warehouse serving a small shop can still be ancillary to the retail use.

Rating and council tax records are evidence but not determinative. So are leases. A case officer will weigh them, and an inspector will look behind them to what actually happens on the ground.

Composite uses and how to describe them

Composite uses cause more trouble at the drafting stage than at the assessment stage, because people try to squash them into a single class.

A rural building used partly for storage of agricultural machinery, partly for a vehicle repair business and partly for open storage of building materials is a composite use. If you describe it as B8 storage you have mis described it, and any certificate you obtain may be worthless. If you describe it as a mixed use with each component identified by area and activity, you have something enforceable and something that can be varied later.

The practical drafting rule is to name the components and, where you can, to quantify them. Floor areas, hours, vehicle movements, staff numbers. A description that says what the mix is can be relied on. A description that says "mixed commercial use" cannot.

Where a composite use includes an element that would be sui generis on its own, that element does not disappear into the mixture. The full picture of what sits outside the classes after 2020 is set out in our guide to sui generis uses after the 2020 reforms.

Splitting a unit and creating a new one

Subdivision is the situation most likely to reach an appeal. Splitting a single planning unit into two or more can itself be a material change of use, even where the activity in each part would be within the same class.

The reason is that the planning consequences change. Two separate retail units generate different servicing patterns, different signage, different hours, and each becomes capable of independent change later. The same logic applies to dwellings: dividing a house into two flats is a change of use requiring permission, because it creates two planning units where there was one.

Conversely, amalgamation can be development too, though it is less often pursued. Combining two flats into one has been treated as a material change of use in some circumstances, particularly where local policy protects the housing stock.

Where a client wants flexibility, the safer route is to apply for the subdivision expressly and to describe the resulting units. Relying on a permitted development right that assumes a single unit is how people end up with an enforcement notice. And where the works have already happened, section 191 and 192 certificates are the formal way to establish what is lawful.

Consequences for permitted development rights

Permitted development rights attach to the planning unit, and this is where the analysis pays for itself.

The flexibility within Class E is flexibility within a single planning unit. It does not authorise splitting a unit into three smaller ones with different Class E uses. Part 3 prior approvals, including the Class MA route from commercial to residential, apply to a building or a unit as defined, and the extent of the unit determines what can move.

Householder rights under Part 1 attach to a dwellinghouse and its curtilage, so an outbuilding within the curtilage is part of the unit. Let that outbuilding separately as a workshop and it may become a unit of its own, at which point the Part 1 allowances no longer cover it.

Storage and distribution rights under Part 3, agricultural rights under Part 6 and the temporary use rights under Part 4 all turn on the same question. If the unit is misidentified, the right does not exist.

Setting the unit out in a supporting statement

Officers should not have to work this out from your drawings. A Planning Statement that deals with the unit properly contains four things:

1. A plan showing the extent of the planning unit, with the red line and any separate areas distinguished.
2. A short factual account of occupation: who occupies what, under what arrangement, since when, with separate access and servicing identified.
3. The Burdle analysis applied, saying which of the three situations the site falls into and why.
4. The resulting description of the existing use, phrased so it could be carried into a certificate or a condition.

Where the existing use is contested, the planning history matters: previous permissions, their descriptions, any conditions restricting use, and any earlier certificates. Pull the file before you write the statement.

The honest uncertainty is that the third Burdle situation is fact sensitive and decisions on similar facts do diverge. Treat appeal decisions as indicative of how inspectors have approached the question rather than as binding precedent, and set out the facts fully enough that an officer can reach your conclusion without your help. For how section 55 handles cases where no development occurs at all, see section 55 and when a change of use is not development.

This is general guidance, not legal advice. A Lawful Development Certificate is the only formal way to confirm that an existing use is lawful, so check the position with your local planning authority or a qualified professional before relying on it.

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