Mixed use and the planning unit: getting the description right

A mixed use planning unit is one where two or more uses run together without either being ancillary to the other, and the consequence is that the site sits outside every class in the Town and Country Planning (Use Classes) Order 1987 as amended. It is sui generis. That means no permitted change applies, the description of development on the decision notice becomes the only definition of what is lawful, and getting that wording wrong costs the client the flexibility they thought they were buying.

Most of the damage is done in the description field on the application form, which is usually filled in last.

Identifying the planning unit

The planning unit is the area against which you ask whether a material change of use has occurred under section 55 of the Town and Country Planning Act 1990. Get the unit wrong and the answer to the change of use question is wrong too.

The approach the courts have taken since Burdle in 1977 gives three situations. Where the whole unit of occupation is used for a single main purpose, with other activities merely incidental or ancillary to it, the unit of occupation is the planning unit. Where two or more physically separate and distinct areas are occupied for substantially different and unrelated purposes, each area is its own planning unit. And where activities of different kinds are carried on within the same occupation without any one being ancillary, the whole is a single unit in composite use.

Occupation matters more than ownership. A landlord who owns a parade of five shops has five planning units, not one, because each is separately occupied. A single operator running a pub with four letting rooms above has one.

Composite use against two separate units

This distinction decides what a later change requires, so it is worth resolving on day one.

Two separate planning units means each has its own use and its own history. Convert the office element to residential and you are dealing with that unit alone. A composite use means the site has a single sui generis use made up of parts, and changing the balance between those parts can itself be a material change of use, even though no new activity has appeared.

The practical test is functional rather than physical. Shared staff, shared servicing, a shared entrance, one till, one set of accounts and one operator point towards composite use. Separate lettings, separate access and separate operators point the other way. Physical separation is evidence, not the answer.

Where a use has ceased for a long period, neither analysis may survive: the lawful use can be lost altogether, and our note on abandonment and how a lawful use can be lost sets out the factors an inspector weighs.

Why the description does the work

For a sui generis mixed use, there is no class to fall back on. The permission is construed by reference to what it grants, and what it grants is the description of development, read with the conditions and the approved plans.

So a description that says change of use to mixed use gives the operator almost nothing to work with, and a description that says change of use to a mixed use of café (Class E(b)) and micro-brewery (sui generis) with ancillary retail sales gives them a defensible position when the environmental health officer or a new owner asks what is permitted.

The wording also constrains later applications. Section 73 varies conditions, and it cannot be used to vary a condition in a way that conflicts with the operative part of the permission, which means the description cannot be rewritten through that route. A non-material amendment under section 96A can correct wording in limited circumstances, but it will not convert one development into another. The practical position is that a poor description is usually fixed only by a fresh application. Why the wording binds you works through the consequences in more detail.

Drafting a description that survives

Write the description as if the reader is a different officer in seven years with no knowledge of the scheme. Four elements do most of the work.

1. Name each use, with its use class in brackets where one applies and sui generis where none does.
2. Quantify the split, by floor area or by unit, where the balance between the elements matters.
3. Say which elements are ancillary, if any, and to what.
4. Describe the physical works separately from the use, because works and use are different things and conflating them produces permissions that cannot be implemented in stages.

Avoid two habits. Do not import marketing language into the description, because flexible workspace hub is not a use and will be construed against the applicant. And do not describe a use by reference to an operator, since the permission runs with the land and the operator will change.

An illustrative form of words for a ground floor unit: use as a mixed use comprising a bakery with on-site consumption (Class E) and a hot food takeaway (sui generis), with the takeaway element limited to the area shown on drawing reference X, together with the installation of an extract flue to the rear elevation. Illustrative only, and real wording should follow the actual scheme and the council's own drafting conventions.

Conditions that tie the elements together

Where the elements have to remain in a particular relationship, that relationship belongs in a condition rather than in hope.

Useful conditions on a mixed use include a floorspace cap on one element, a restriction that one element is operated only in conjunction with the other and by the same operator, opening hours by element, a restriction on subdivision into separate planning units, and delivery or servicing controls. Each has to meet the six tests, so the reason for the condition must be a planning reason and it has to be enforceable in practice.

Be careful what you ask for. A condition restricting the use to a named class, or preventing the change that the Order would otherwise permit, is a real restriction on value, and our piece on conditions restricting use class or occupier covers where those conditions can be defended and where they cannot.

Changing one element later

This is the question clients ask eighteen months after permission is granted, usually on a Friday.

If the site is a composite sui generis use, replacing one element with another is a change to the composite use itself and normally needs permission, even where the new element would fall inside Class E on its own. The Class E flexibilities do not rescue a sui generis use. Intensifying one element to the point where the character of the whole changes can also be material, although that argument turns on evidence of degree rather than on the words in the description.

If the elements are separate planning units, each moves under its own rules and permitted changes apply to each independently. That is usually the better outcome for a landlord, and it is worth designing for at the outset through separate access, separate services and separate lettings.

Errors that show up on the form

Four recur often enough to be worth a checklist before submission. Describing a true composite use as a single class, which invites a validation query at best. Leaving the floorspace split out where the balance is the whole point. Describing the works but not the change of use, so the permission authorises a flue and nothing else. And using the existing use description from the previous decision notice without checking whether the use has since changed.

Set the description out in the Planning Statement as well as on the form, with a short explanation of why the site is a single unit in composite use or two units, and the supporting evidence pack for a change of use application should carry the occupation history that backs it up.

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.