The planning unit and why it decides your change of use

Before you can say whether a change of use is material under section 55 of the Town and Country Planning Act 1990, you have to say what the change is a change of. That is the planning unit question, and it decides more change of use cases than the Use Classes Order does. Get the unit wrong and the whole analysis collapses: an activity that looks like a material change within a small unit is often an ancillary activity within a larger one, needing no permission at all.

The doctrine is judge-made rather than statutory, which is why it rewards careful reading and punishes assumption.

What a planning unit is

A planning unit is the parcel of land by reference to which the existing use is identified and against which any change is measured. It is not the same as the title boundary, the rateable unit or the lease.

The classic starting point comes from the approach set out in Burdle v Secretary of State for the Environment, which gave three situations. Where the occupier uses the whole of a unit of occupation for a single main purpose, with other activities incidental or ancillary to it, the unit of occupation is the planning unit. Where the occupier carries on a variety of activities and none is ancillary to another, the unit of occupation is still generally the planning unit. But where within a single unit of occupation there are physically and functionally separate areas each used for a substantially different and unrelated purpose, each area may be a separate planning unit.

That third situation is the one that does the work in practice, and it is the one most often argued about.

The unit of occupation as the starting point

Start with occupation, because the courts do.

Ask who occupies the land, on what terms, and whether the occupier's activities across it hang together. A farm occupied by one farmer is usually a single unit even though it contains a house, buildings, yards and fields. A shop with a flat above, both occupied by the shopkeeper, is usually one unit. Split the occupation, by letting the flat separately on its own tenancy with its own access, and you have created two units, and the answer to every subsequent question changes.

The practical corollary matters for anyone advising on a portfolio. Granting a lease over part of a building can create a new planning unit without any physical works, and can therefore create a change of use, entirely inadvertently. That is a real risk when a small part of a warehouse is let to a separate trade operator.

Where separate activities form separate units

To make a separate unit within a larger holding, an area normally needs both physical and functional separation.

Physical separation means something a visitor could see: its own entrance, a boundary, a distinct building or a defined and delimited area. Functional separation means the activity does not serve the main use. A staff canteen inside a factory serves the factory, so it is ancillary, however well defined the room is. A cafe in the same factory selling to the public, with its own street door and its own hours, is doing something substantially different and unrelated, and is a candidate for a separate unit.

Neither limb alone is enough. A physically distinct room used for a purpose that serves the main use stays ancillary. An unrelated activity carried on in a corner of an open workshop, with no physical definition, usually stays part of the single unit until it grows.

Related questions run alongside this, particularly whether increasing an existing activity crosses the line on its own, which the piece on intensification and when doing more of the same needs consent works through.

Why the unit changes the answer on materiality

Materiality is assessed as a matter of fact and degree, by reference to the planning unit. Change the unit and you change the denominator.

Take a 2,000 m² industrial building in a single occupation, with 150 m² given over to a trade counter serving the public. Assessed against the whole building, the trade counter is a small ancillary part of a general industrial use, and no permission is needed. Assessed as its own unit, the 150 m² is retail, which is a different use altogether and clearly a material change. Same floor area, same activity, opposite legal result. This example is illustrative, but it is the pattern behind a large share of enforcement disputes.

The same logic runs the other way. Where an applicant wants to argue that a proposed activity is ancillary and needs no permission, the case rests on showing a single unit with a single main purpose. Where an authority wants to argue a breach, it usually argues for a smaller unit.

Section 55(2)(d) gives a related rule for dwellings: use of any buildings or land within the curtilage of a dwellinghouse for a purpose incidental to the enjoyment of the dwellinghouse is not development. Incidental is a narrower idea than ancillary, and it is the reason a home office is usually fine and a commercial workshop in the same shed usually is not.

Evidence that defines the unit

The unit is a question of fact, so evidence decides it. Assemble the following before you form a view.

1. Title plans, leases and licences, showing who occupies what and on what terms.
2. A measured floor plan with areas by activity, not by room name.
3. Photographs of entrances, signage, boundaries and internal divisions.
4. Business rates records, which record the rateable unit and are indicative rather than conclusive.
5. Staffing and management arrangements, including whether the areas share staff and supervision.
6. Turnover or throughput by activity, where the issue is whether one activity has outgrown its ancillary status.
7. Historic aerial photography and street imagery to show how the arrangement has changed over time.

Rating records are the evidence people lean on hardest and should trust least. Separate rating assessments are a useful pointer to separate occupation, but the rating and planning tests are different, and an inspector will say so.

Getting the unit agreed before you apply

The worst place to discover a disagreement about the planning unit is in a committee report, because by then the officer has written a position and the recommendation follows it.

Two routes work better. A pre-application enquiry that sets out the unit, the evidence for it and the conclusion that follows, and asks the officer to confirm or challenge the unit specifically rather than the proposal generally. Or an application for a lawful development certificate under section 191 or 192 of the 1990 Act, which puts the question directly and produces a binding answer on the facts presented. The certificate route costs more and takes longer, and on a site where a future sale or funding depends on the answer it is usually money well spent.

Where you do apply for permission, deal with the unit explicitly in the Planning Statement rather than assuming the officer shares your view. A short section identifying the unit, the evidence, the main use and the ancillary activities gives the case officer something to agree with, and it gives an inspector a starting point if the case goes further. The related analysis of how councils decide what they assess covers the officer-side view of the same question.

One honest uncertainty is worth stating. The case law gives a framework rather than a formula, and two experienced planners can reach different conclusions on the same facts, particularly where an activity has grown gradually over years. Where that is the position, say so in the statement and set out both readings. An inspector is far more receptive to an argument that acknowledges the difficulty than to one that pretends the answer is obvious.

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

Where the change of use raises trip generation or servicing, a Transport Statement is often the document the highway officer asks for first, and PlanScope can draft one for review.

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