The planning unit and when a mixed use arises
Identify the planning unit before anything else. It decides the description of development, whether an activity is ancillary, and whether a later change needs permission.
Before you can say whether there has been a material change of use, you have to decide what the thing is that has changed. That thing is the planning unit, and identifying it correctly is the whole of the planning unit mixed use question. Get the unit wrong and everything downstream goes wrong: the description of development, the use class, whether a second activity is ancillary or a use in its own right, and whether a later change needs permission at all.
Section 55 of the Town and Country Planning Act 1990 makes a material change in the use of any buildings or other land development. It does not define the land. Case law does.
The tests that identify the unit
The starting point remains Burdle v Secretary of State for the Environment (1972), which set out three situations.
Where the whole of a unit of occupation is used for a single main purpose, with other activities incidental or ancillary to it, the unit of occupation is the planning unit. A farm with a farm shop selling its own produce is the obvious case.
Where two or more activities are carried on in different parts of a unit of occupation, and each part can be identified physically, each of those parts may be a separate planning unit. A ground floor shop with self-contained offices above, reached by their own door, is likely to be two units.
Where two or more activities are carried on throughout the unit without any physical separation, and neither is ancillary to the other, the unit is in mixed or composite use.
Occupation is the organising idea in all three. The unit is normally the area in one occupier's control, not the registered title and not the planning application site. A tenant occupying two adjoining shops as one business usually has one planning unit, while a landlord letting two parts of the same building separately usually has two.
Physically separate parts of one building
The second limb of Burdle does the most work in practice and causes the most argument. Officers look for separation you can see and use: a separate entrance, no internal connection, separate services and metering, a separate postal address, separate rating assessments, and a lease of the part rather than the whole.
None of those is decisive on its own. A separate rating assessment is evidence, not proof, because the rating and planning systems ask different questions. The internal door is often the pivot. A flat above a shop reached only through the shop is harder to treat as a separate unit than the same flat with a street door, which is why the question of whether upper floors above a shop are a separate planning unit turns up so often on conversion schemes.
Where you are advising on a building in multiple occupation, map the units before writing anything else. Draw them. Colour them. Then write the description.
When a second activity stops being ancillary
An ancillary use serves the primary use and has no independent existence. The staff canteen in a factory, the office in a builders' yard, the storage building behind a shop. Ancillary activities do not need their own permission and do not appear in the description of development.
Three things push an activity out of ancillary status.
- It serves customers or users who are not there for the primary use. A factory canteen open to passing trade is no longer ancillary.
- It grows until the primary use no longer dominates. There is no percentage threshold, which is the honest answer, although floorspace, turnover, staff time and visitor numbers are all used as evidence.
- It becomes functionally independent, with its own staff, hours, entrance and identity.
The consequence matters. If the second activity is a use in its own right and nobody obtained permission for it, you have an unauthorised mixed use and a potential enforcement exposure, whatever the rateable description says. That is a particular risk inside Class E, where the flexibility between sub-classes is sometimes read as flexibility to do several things at once. Our note on light industrial and office uses inside Class E(g) sets out where that reading breaks down.
Describing a mixed use correctly
A mixed use is not the sum of two use classes. It is a single composite use, and the description has to say so.
| Situation | Correct description |
|---|---|
| Shop with a workshop at the back serving the shop | Use as a shop, workshop ancillary |
| Shop and an independent café in the same open space | Mixed use as shop and café |
| Office with a storage yard used by a separate business | Two planning units, described separately |
| Dwelling with a consulting room used by visiting clients | Mixed use as dwelling and consulting room |
Write the description in the terms of the activities, not the class letters. "Mixed use falling within Classes E(a) and E(b)" is a description nobody can enforce, because the classes allow movement and the mix would be free to change. "Mixed use as a retail shop and a café with 24 covers" tells everyone what was permitted.
Give the proportions where they matter. If the café is acceptable at a third of the floorspace and unacceptable at two thirds, the permission needs to say which was assessed.
Conditions on a mixed use permission
Because a composite use is a single use, the ordinary rule that no permission is needed to change within a use class does not help the council. A condition is the only way to hold the balance where the balance matters.
The conditions that survive are precise and plan-based. Restricting a named activity to an area shown on a submitted plan. Capping the floorspace or the number of covers. Fixing hours for the element that affects neighbours. Requiring extraction to the approved specification where food is cooked, which is where a Ventilation and Extraction Statement earns its place. Restricting servicing hours or vehicle sizes where the mix changes the trip pattern, supported by a Transport Statement scoped to the mix rather than to one element.
Conditions that fail are the vague ones. A condition requiring the use to remain "predominantly retail" invites an argument about what predominantly means, and an inspector will usually delete it.
What the unit decides about later changes
Once a mixed use is established, the use of the whole unit is the composite use. A change in the internal balance can itself be a material change of use, because the composite use has changed character. That is why a permission for a mixed use is often more restrictive in practice than two permissions for two units.
The reverse is also true, and it is useful. Where a building is correctly two planning units, each can move within its own class independently. Splitting a unit is itself capable of being development, so a scheme that creates a new self-contained part should be applied for rather than assumed.
An illustrative example. A vet occupies a converted double-fronted shop. Consulting rooms, a dispensary and a small retail display of pet food sit in one open space with one entrance, under one lease. There is one planning unit in mixed use, with the retail display ancillary while it stays a display. If the owner later lets the left-hand half to a separate pet shop with its own door and services, two units emerge and the description of each has to be revisited. A Planning Statement that sets out the unit analysis, the evidence for it and the proposed description saves the officer from reaching their own view on incomplete facts.
And that is the practical reason to do this work first. The unit analysis is the one part of a change of use case you cannot correct later by amendment.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Where certainty about an existing use is needed, a Lawful Development Certificate under section 191 or 192 of the 1990 Act is the only formal route. PlanScope produces AI-assisted draft reports for planning applications, and reports should be reviewed before submission. PlanScope does not guarantee planning approval.
Frequently asked questions
What is the planning unit?
It is the area of land or building treated as a single entity when deciding whether a material change of use has occurred. Burdle v Secretary of State for the Environment (1972) ties it primarily to the unit of occupation, with separate physical parts of a building capable of being separate units. It is not the title boundary or the application site.
How do I tell whether a use is ancillary or part of a mixed use?
An ancillary use serves the primary use and has no independent existence. Once an activity serves its own customers, has its own staff, hours and entrance, or grows until the primary use no longer dominates, it becomes a use in its own right. There is no percentage threshold, so floorspace, turnover and visitor numbers are used as evidence.
Can a condition control the balance within a mixed use?
Yes, and usually it has to, because a composite use is a single use and changes within it would otherwise be unrestricted. Workable conditions tie a named activity to an area on a submitted plan, cap floorspace or covers, and fix hours. A condition requiring the use to remain predominantly one thing is too vague to enforce.
Is dividing a building into two units development?
It can be. Creating a new self-contained unit changes the planning unit and may amount to a material change of use, particularly where services, access and occupation are separated. Rather than assuming the split is free, apply for it or obtain a certificate of lawfulness, because the point is frequently raised later on sale.
Does a separate business rates assessment prove a separate planning unit?
No. Rating and planning ask different questions, so a separate assessment is evidence rather than proof. Councils look at the whole picture: separate entrances, absence of internal connection, separate services and metering, separate leases and how the parts are actually occupied day to day.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.