The planning unit and how its boundary is drawn

The planning unit meaning that matters in practice is this: the area of land treated as one entity when deciding whether a material change of use has taken place under section 55 of the Town and Country Planning Act 1990. Draw the unit wide and an activity looks like a subsidiary part of a larger use. Draw it narrow and the same activity becomes a use in its own right, needing permission.

Nothing in the 1990 Act defines it. The concept comes from case law, and the formulation everyone still uses is Bridge J's in Burdle v Secretary of State for the Environment.

Why the boundary decides the answer

A material change of use is assessed by comparing the use of the planning unit before and after. So the unit is the denominator in every change of use question, and moving it moves the result.

Consider a farm where one barn starts being used for vehicle repairs. If the planning unit is the whole holding, the argument is about whether the character of the use of the farm as a whole has materially changed, which on a large holding it may not have. If the barn and its yard are a separate planning unit, the argument is about whether a barn has changed from agricultural use to a repair workshop, which it plainly has.

Same facts. Different unit. Opposite conclusion. This is why the unit is argued first in enforcement appeals and lawful development certificate applications, and why a planning statement that asserts the unit without reasoning leaves its main point undefended.

The tests used to identify one

Burdle sets out three situations, and they work as a sequence rather than as alternatives to pick from.

Where the unit of occupation is the same as the unit of use, and the whole is used for a single main purpose with other activities incidental or ancillary to it, the whole unit of occupation is the planning unit. A factory with a staff canteen, a car park and an office block is one unit in one use.

Where the occupier carries on a variety of activities within one unit of occupation, none of them ancillary to any other, and it is not possible to identify a single main purpose, the unit of occupation is still the planning unit and the use is a composite or mixed use. A builder's yard with storage, retail sales and vehicle maintenance, all run together, is one unit in mixed use.

Where within a single unit of occupation there are two or more physically separate and distinct areas occupied for substantially different and unrelated purposes, each area is a separate planning unit. That is the exception that does the most work, and the one most often argued badly.

Occupation, physical separation and function

Three strands of evidence decide which of the three situations applies.

Occupation is the starting point, and the default. Who occupies the land, under what interest, and does the occupation of one part depend on the other. A separate lease is evidence of a separate unit, though not conclusive.

Physical separation asks whether the areas are marked out on the ground: a fence, a wall, separate access, separate services, a hedge line. A notional line on a plan is not physical separation. Officers and inspectors look for something a visitor would notice.

Function asks whether the activities serve each other. A canteen serving the factory workforce is functionally ancillary. A canteen open to the public, with its own entrance from the street, has stopped being ancillary and has probably become a separate unit.

None of the three is decisive alone. An inspector weighing them will usually start with occupation and only depart from it where separation and function both point the other way.

Multiple units within one title

A registered title tells you about ownership rather than about planning. One title can contain several planning units, and one planning unit can span more than one title where the occupation is shared.

This trips up applications regularly. The red line on the application drawing is the application site, which the applicant chooses. The planning unit is a matter of fact and degree, which the applicant does not. Where the two differ, say so in the supporting statement rather than hoping the difference goes unnoticed.

The blue line, showing other land in the applicant's control, is sometimes the clue that the planning unit is wider than the red line. An officer who sees a small red line inside a large blue line will ask the question.

How subdivision creates new units

Subdividing is the clearest way to create planning units, and section 55(3)(a) of the 1990 Act removes any doubt for dwellings: the use as two or more separate dwellinghouses of a building previously used as a single dwellinghouse involves a material change of use of the building and of each part of it.

Beyond dwellings, subdivision is assessed on the ordinary tests. Splitting a large retail unit into two shops with separate frontages, separate tills and separate tenancies creates two units, though whether that amounts to development at all depends on whether the use of each is materially different from what went before.

The reverse also happens. Amalgamating two units into one, whether two flats into a house or two shops into a store, can be a material change of use, and several London boroughs have policies aimed squarely at the loss of smaller homes through amalgamation.

Intensification is the harder case. More of the same activity within the same unit is usually not a material change, until the character of the use changes. There is no threshold, no percentage and no shortcut. It is a matter of fact and degree, which is an unsatisfying answer that happens to be the correct one.

Effect on whether a material change has occurred

Once the unit is settled, the comparison is between its use before and after, judged by the character of the use rather than by labels.

The middle row carries a practical warning. A composite use is defined by the mix as it stood at the relevant date, so a significant shift in the balance between components can itself be a material change even though every component was already present.

Evidencing the unit in an application

Show the unit rather than assert it. That means a plan marking the boundaries you say apply, with the evidence for each: title extent, lease boundaries, physical features on the ground, access arrangements, service connections and the functional relationship between areas.

For a lawful development certificate the evidential burden sits on the applicant, and the standard is the balance of probabilities. Statutory declarations, dated photographs, utility bills, business rates records and accounts all help. Vague recollections do not.

A Planning Statement is the right place to set out the unit and the reasoning, before the policy assessment begins, because the policy assessment depends on it. Where a change of use is proposed, the supporting documents the council expects will follow from that analysis, and the full list of report types by category is a reasonable starting point for scoping the pack.

Enter your postcode on PlanScope to see which reports a site is likely to need, or create a free account to draft the planning statement and work the unit analysis into it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.