Planning unit: the piece of land the council assesses

The planning unit is the parcel of land or building the local planning authority treats as one entity when deciding whether a material change of use has happened. It is normally the unit of occupation: the whole of the area in one occupier's control, used for a single main purpose, with any lesser uses treated as ancillary to that purpose. The planning unit definition matters to landlords and conversion investors because everything else, whether you have gone from C3 to C4, whether Class MA applies, whether you need permission at all, is measured against it.

Get the unit wrong and the argument is over before it starts. Argue that a rear annexe is a separate dwelling when the council treats the whole house as one unit, and you are arguing about the wrong thing.

Why the unit comes before the use

Section 55 of the Town and Country Planning Act 1990 makes development include "the making of any material change in the use of any buildings or other land". Change from what, to what, and over what area? The Act does not say. The courts filled the gap with the planning unit.

So the sequence is always the same. Identify the unit, identify its use before the works, identify its use after, then ask whether the difference is material in planning terms. Councils and inspectors do it in that order, and a lawful development certificate application that skips step one usually gets refused.

The practical consequence for a landlord is scale. If the whole house is one planning unit in C3 use and you let it to five sharers, you have changed the use of that one unit to C4 (or a sui generis large HMO above six people). If instead each floor were a separate planning unit already in residential use, the analysis is different again.

The three situations the courts identified

The leading statement is Burdle v Secretary of State for the Environment [1972], where the Divisional Court set out three broad situations. It is often called the Burdle test, though it reads more as guidance than a formula.

First, where the occupier's whole holding is used for one main purpose, with other activities incidental or ancillary to it, the whole holding is one planning unit. A house with a home office in the back bedroom is one unit in residential use.

Second, where the occupier carries on two or more activities that are not ancillary to one another, but they are not physically separated, the whole is still normally one unit with a composite or mixed use. A yard used partly for storage and partly for vehicle repairs, with no fixed line between them, is a single unit in mixed use.

Third, where within a single holding there are physically separate and distinct areas occupied for substantially different and unrelated purposes, each area may be a separate planning unit. This is the limb that conversion investors rely on, and it is the hardest to establish, because it needs both physical separation and functional separation.

The court was clear that the tests are practical rather than legalistic. The question is what a sensible observer would treat as the unit of occupation, not what the title deeds say.

Occupation and ownership are different things

Ownership is a poor guide. A landlord who owns a terrace of six houses does not own one planning unit; there are six, because there are six separate occupations. Equally, a single owner-occupier who has knocked two cottages into one house may have created a single unit even though two titles remain.

What the council looks at, broadly, is:

• who has control of the space and who can exclude others from it
• whether there is a separate entrance and whether internal doors are locked or open
• whether kitchens, bathrooms and living space are shared or self-contained
• whether services, meters, bins and post are shared
• whether council tax and tenancy documents treat the space as one dwelling or several

None of these is decisive on its own. A separate front door does not create a separate planning unit if the occupiers still live as one household, and a shared meter does not prevent one where two families live entirely independently.

This is also where the planning unit differs from curtilage. Curtilage is about the land that serves a building, and it drives permitted development limits under the GPDO 2015. The planning unit is about the entity whose use is being assessed. The two often cover the same ground, but they answer different questions, and an argument that mixes them up tends to unravel on appeal.

Splitting a building into separate units

Subdivision is itself development. The 1990 Act says at section 55(3)(a) that the use as two or more separate dwellinghouses of a building previously used as a single dwellinghouse involves a material change of use. So converting a house into flats needs planning permission, full stop, however modest the physical works.

Going the other way, amalgamating flats back into one house, is not covered by that subsection, so it depends on whether the change is material on the facts. Several London boroughs have policies resisting the loss of smaller homes, and in a borough with such a policy the council may well argue materiality. Check the adopted local plan before assuming amalgamation is free of planning control.

For HMOs, the planning unit question is whether the house remains one unit occupied by a household or group of sharers, or whether you have created bedsits that function as separate units. A property with individually locked rooms, each with its own cooking facilities, no shared living room and separate tenancies starts to look like several planning units rather than one C4 house. That matters, because Class C4 covers a single dwellinghouse occupied by between three and six unrelated people sharing basic amenities. Lose the sharing and you may be outside C4 altogether, in sui generis territory, where the permitted development right in Schedule 2 Part 3 Class L of the GPDO 2015 (C3 to C4) does not help you.

And where an Article 4 direction removes the C3 to C4 right, as many university towns and London boroughs have done, you need a full application for the change of use regardless. Verify the extent of any direction on the council's own mapping, because directions are often drawn around specific wards rather than the whole authority.

Class MA and Class Q

Prior approval routes carry their own unit questions. Class MA (commercial, business and service use in Class E to dwellinghouses in C3) applies to a building, or part of a building, that was in Class E use for a continuous period of at least two years before the application. If the ground floor shop and the first floor storage are one planning unit in Class E use, the two-year test is applied to that unit. If the upper floor has been a separate residential unit for years, it was never in Class E use and Class MA does not reach it.

Class Q (agricultural buildings to dwellinghouses) turns on the building being part of an established agricultural unit on 20 March 2013. Here the relevant unit is the agricultural unit, which is a defined term in Part 3 of the GPDO 2015 rather than the general planning unit, so read the class conditions rather than reasoning from first principles.

Illustrative example: a shop with a flat over

This example is illustrative only. A high street property has a hairdresser at ground floor and a one-bedroom flat above, reached by a separate door from the street. The freeholder occupies the salon and lets the flat on an assured shorthold tenancy.

There are two planning units here. The salon is one, in Class E(b) or E(d) use depending on the services offered. The flat is another, in C3 use. They are physically separate, separately occupied and used for unrelated purposes, which is Burdle's third situation.

Now the freeholder wants to use the flat as a treatment room ancillary to the salon. That is a change of use of the upper planning unit from C3 to a Class E use, and it needs permission, even though nothing changes outside and the same person controls both floors. Councils in areas with policies protecting existing housing stock frequently resist exactly this.

Flip the facts. If the flat had only ever been used as staff accommodation, accessed through the salon, with no separate tenancy, the two floors would probably be one planning unit in mixed retail and residential use. Then converting the upper floor to treatment rooms might be no more than a change in the internal balance of an existing mixed use, and arguably not material at all. Same building, different unit, different answer.

Getting the unit right in a lawful development certificate

If you have inherited a building where the lawful use is unclear, an application under section 191 of the 1990 Act for a certificate of lawfulness of existing use or development is the formal way to settle it. Sections 191 and 192 certificates are the only mechanism that binds the council.

A workable sequence:

1. Define the red line on your plan around what you say is the planning unit, and say why on Burdle grounds.
2. Evidence occupation over the relevant period with tenancy agreements, council tax records, utility accounts, electoral roll entries, insurance and photographs.
3. State the use of that unit at the start of the period and at the date of application, using use class references where they apply.
4. Deal with materiality expressly: what changed, when, and why it was or was not material.
5. Address the time limits. Ten years is now the general immunity period for change of use in England following the Levelling-up and Regeneration Act 2023 amendments to section 171B, with four years retained for a single dwellinghouse in limited circumstances; check the current position for your facts before relying on either figure.

The burden of proof is on the applicant, and it is the civil standard, balance of probabilities. Sworn statements help, but documents that were created at the time carry more weight than recollection.

Where the unit boundary is genuinely arguable, say so in the statement and give the council the alternative. Officers are more likely to engage with a candid case than with an overstated one. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

The same discipline pays off in a full application. A Planning Statement that opens by defining the planning unit, then works through the use before and after, gives the case officer the structure they need. If you also want the design analysis to hold up, the way you identify the principal elevation follows a similarly fact-specific logic.

Enter your postcode on PlanScope to see which reports your site is likely to need before you draft the change of use case.

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