The planning unit and when one building holds two uses
Identifying the planning unit decides whether a building in two occupations has one composite use or two, which permitted development rights apply, and when immunity starts to run.
The planning unit is the area of land against which a use is characterised, and identifying it correctly decides three practical questions: whether a building in two occupations has one mixed use or two separate uses, which permitted development rights are available, and when immunity from enforcement starts to run. Get it wrong and a prior approval application goes in for the wrong building, or a certificate application describes the wrong land.
The leading guidance comes from Burdle v Secretary of State for the Environment (1972), where the court set out how to approach the question rather than laying down a formula. The starting point is the unit of occupation. Where the whole of that unit is used for a single main purpose, with other activities ancillary to it, the unit of occupation is the planning unit. Where physically separate and distinct areas within it are occupied for substantially different and unrelated purposes, each of those areas is a separate planning unit. And where activities of different character are not confined to separate areas, the right characterisation may be a single composite or mixed use.
How the unit is identified in practice
Occupation comes first, geography second. A single occupier of a whole building with an office on the ground floor and storage above has one planning unit with an ancillary use, even though the activities differ.
Two occupiers of the same building is where the analysis starts to work. The questions officers and inspectors ask are consistent.
- Is each area physically separate, with its own entrance and no internal connection in normal use?
- Is each in separate occupation under its own lease, licence or tenancy?
- Are the activities unrelated, or does one serve the other?
- Does each area function independently, with its own services, staff and customers?
Where the answers point the same way, two units is the right answer. Where they conflict, the activity usually carries more weight than the paperwork. A lease dividing a unit on paper, with staff and customers moving freely between the two halves, will not create two planning units.
Mixed use against two distinct units
The distinction is not academic. A mixed use is a single use of a single unit, described as a composite, and it is lawful as a whole. Two separate units each have their own lawful use, and each can change independently.
Take a building with a shop at the front and a tattoo studio at the back, one occupier, customers passing between them. That is one unit in composite use, part Class E(a) and part sui generis. The owner cannot later change the shop half without considering the whole, because there is no "shop half" in planning terms.
Now give the back half its own street door, its own lease and its own tenant. Two units. The front can move freely within Class E while the back remains sui generis, and a change to the back has no bearing on the front. Our note on splitting a commercial unit into two deals with how that division is achieved and what permission it needs.
Residential is the exception where the statute does the work for you. Section 55(3)(a) of the Town and Country Planning Act 1990 deems the use as two or more separate dwellinghouses of a building previously used as a single dwellinghouse to be a material change of use. No Burdle analysis is needed: subdivision of a house into flats always requires permission.
What it changes for permitted development
Most permitted development classes are drafted by reference to a building or a dwellinghouse, which means the unit determines what the right applies to.
| Scenario | Consequence |
|---|---|
| Class MA prior approval | Requires the building to have been in Class E use, so a building in composite use including a sui generis element may not qualify |
| Part 1 householder rights | Apply to a dwellinghouse, not to a flat, and not to part of a building in mixed use |
| Class A shop to dwelling style rights | Depend on the existing lawful use of the relevant part, so the unit boundary decides eligibility |
| Changes within Class E | Only available where the relevant part is itself a planning unit in Class E use |
A building in composite use is the trap. Where the lawful use is a single mixed use, there is no separate Class E use to move within and no separate agricultural building to convert. Applicants discover this when a prior approval is refused for a reason that has nothing to do with the merits.
Our note on the workshop in a garden illustrates the residential version of the same problem, where a business building in a garden can become a second planning unit and take the land it sits on out of the dwellinghouse.
What it changes for enforcement and immunity
Time runs against a breach in a planning unit. Where the unit changes, the clock can restart, because what is being assessed is a different use of different land.
Intensification is the live issue. A use that grows until it changes the character of the whole unit can amount to a new material change of use, with a fresh period of immunity to run. That cuts both ways: an owner relying on long use for a certificate has to show the use as it exists now has subsisted for the relevant period, not merely that something similar started long ago.
The time limits themselves were altered by the Levelling-up and Regeneration Act 2023 in England, so check the current position for the breach you are dealing with rather than working from the four and ten year rules as they stood before.
An illustrative example
This example is illustrative. A detached building on a village street has a hairdresser at the front, occupied under a lease since 2014, and a workshop at the rear used by the owner for furniture restoration since 2016, with an internal door that has been locked throughout.
The front is a planning unit in Class E(a) use, lawful on its original permission. The rear is arguably a separate unit in a Class B2 or sui generis use, depending on the processes, with a lawful use only if immunity has accrued.
The owner wants to convert the rear to a dwelling. If the rear is a separate unit with a lawful commercial use, that is one application. If the whole building is a single composite unit, the application has to address the whole, and the hairdresser's lease becomes relevant to the planning analysis. The locked door, the separate entrances and the separate occupation all point to two units. The same composite use question arises when a church adds a cafe, which we look at in a church cafe and the use it creates.
Evidence that settles it
Collect documents that show occupation and control rather than documents that describe intentions.
- Leases, licences and tenancy agreements with dates, including any that were granted and never implemented.
- Business rates records for each part, which show separate assessment.
- Utility accounts and meter arrangements.
- Dated photographs of entrances, internal doors and signage.
- Staff and customer arrangements: who answers which door, who has keys, whether facilities are shared.
- Accounts or invoices showing whether one activity supplies the other.
Where the position matters to a transaction or to a prior approval, apply for a certificate rather than argue it later. A certificate of lawful existing use or development under section 191 of the 1990 Act fixes the lawful use of the land described in the application, and a certificate for proposed use under section 192 confirms the position before works start. Either way, a certificate is the only formal way to establish that a use is lawful, and defining the land correctly in the application is the whole exercise.
Browse the 66 report types to see which documents a change of use application usually carries once the unit question is settled.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What is a planning unit?
The area of land against which a use is characterised. The starting point is the unit of occupation, so a single occupier using a whole building for one main purpose with ancillary activities has one planning unit. Physically separate areas in separate occupation used for unrelated purposes are separate units, each with its own lawful use.
How do I know if my building is one mixed use or two uses?
Look at physical separation, separate occupation, whether the activities are related, and whether each part functions independently with its own entrance, services and customers. Where those point the same way the answer is clear. Where the paperwork says two but staff and customers move freely between both halves, the activity usually prevails.
Does subdividing a house into flats always need permission?
Yes. Section 55(3)(a) of the Town and Country Planning Act 1990 deems the use as two or more separate dwellinghouses of a building previously used as a single dwellinghouse to be a material change of use. No analysis of the planning unit is required, and no permitted development right covers it.
Why does the unit matter for a Class MA application?
Class MA requires the building to have been in Class E use. Where the lawful use is a single composite use that includes a sui generis element, there is no separate Class E use to convert, and the prior approval will fail for a reason unconnected to the merits of the scheme. Establish the unit before applying.
Can a change in the planning unit restart the enforcement clock?
It can. Where intensification or reorganisation amounts to a fresh material change of use of the unit, a new period of immunity begins. Anyone relying on long use for a certificate has to show the use as it now exists has subsisted for the relevant period. The time limits in England were altered by the Levelling-up and Regeneration Act 2023.
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.