Two planning units in one building: how to tell

Whether a building holds one planning unit or two is the question that decides what you can do without permission. Separate planning units in one building each have their own lawful use, their own permitted development rights and their own curtilage, so a shop with a flat above may be a single mixed-use unit with one set of consequences, or two units with a quite different set. Nothing on the deeds tells you which. Nothing on the rating list decides it either.

For a landlord buying a mixed building, this is the first analysis to do and the one most often skipped.

How separate units arise

Three routes, in rough order of frequency.

By occupation. A building let as two separate tenancies, each occupied independently for its own purpose, with its own entrance, will usually be two units even where the title is single. The shop and the flat above with separate tenants is the archetype.

By permission. A permission granted for subdivision, or for the erection of a building containing a commercial unit and dwellings, creates the units as approved. The decision notice and the approved plans are then the starting point.

By physical alteration followed by use. Walls built, a stair blocked off, services split, and the two parts used independently for long enough. This is where disputes arise, because the alteration may itself have needed permission and the resulting use may never have been authorised.

What does not create a separate unit: a separate council tax or rating entry on its own, a separate utility meter on its own, or a lease dividing a demise on paper while the whole is used as one operation.

The Burdle approach

The standard framework comes from Burdle v Secretary of State for the Environment, and the three limbs are worth knowing in order.

First, where possible, look at the unit of occupation, meaning the whole area occupied as a single holding, provided the activities carried on there are not confined to any specific part of it.

Second, where within a single unit of occupation there are two or more physically separate and distinct areas occupied for substantially different and unrelated purposes, each of those areas is a separate planning unit.

Third, where there is a single main purpose to which other activities are incidental or ancillary, the whole is one planning unit used for that main purpose.

That third limb does a lot of work in practice. A shop with a stockroom, a staff kitchen and a flat occupied by the manager as part of the business may be one unit, used as a shop, because the residential element is ancillary to the main use. Let that flat to an unconnected tenant and the analysis changes, because the residential use is no longer ancillary to anything.

Physical and functional separation

The two limbs that matter in a real building are physical separation and functional relationship. Test them separately.

Weigh them together rather than counting them. A flat with its own street door, its own meters and an unrelated tenant is a separate unit even if the yard is shared. A flat with its own street door occupied by the shopkeeper and used as part of the business is a harder case, and the answer turns on how far the residential use is independent.

The sealed internal door is the detail that most often decides matters on site. An opening that has been plasterboarded over but can be reopened in an afternoon reads differently from one that has been bricked up and plastered on both faces, and officers do look.

Separate access and services

If you are creating units deliberately, invest in the access and the services rather than in the paperwork.

An independent street entrance, with its own letterbox and its own number, is the single strongest indicator. Separate metered gas, electricity and water follow. Separate refuse storage, assigned to each unit and shown on a plan, helps. A separate postal address registered with the local authority helps more than people expect, because it is a record generated by a third party.

Conversely, where you want to keep a building as a single unit, avoid creating the indicators. Independent entrances and separate meters installed for convenience can be used against you later by an authority arguing that a change of use has already occurred.

What it changes for permitted development

This is where the money is.

Part 1 of the General Permitted Development Order 2015 grants rights to a dwellinghouse, and the Order excludes flats from those rights. So a two storey building that is one planning unit used as a single dwellinghouse has rear extension rights under Class A, roof alteration rights under Class B and C, outbuilding rights under Class E and hard surfacing rights under Class F. Read the same building as a shop unit plus a flat, and the residential part has none of them.

Curtilage follows the unit. Where the rear yard belongs to the commercial unit, Class E outbuilding rights cannot be exercised there by the residential occupier at all, which is the trap behind most disputes about workshops in a residential curtilage.

Commercial rights in Part 7 and Part 3 attach to the commercial unit and are measured against that unit's floorspace rather than the whole building. Where two commercial units exist, each has its own allowance, which can be an advantage.

What it changes for change of use

Changes of use are assessed against the planning unit, so the unit boundaries decide whether anything has changed at all.

Section 55(3)(a) of the 1990 Act provides 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 subdividing a house into flats needs permission, with or without building work, and creating a new unit by any route has to be authorised.

Class MA in Part 3 permits a change from Class E commercial use to residential use subject to prior approval and a set of conditions. Those conditions have been amended since the class was introduced, including the position on floorspace limits and vacancy, so read the current wording of Schedule 2 Part 3 Class MA before relying on it. What matters for planning units is that the class applies to the building or part of the building in Class E use, and the extent of that part is a question about the unit.

Movement within Class E applies to the unit as well. A single mixed planning unit is not a Class E unit, so the flexibility to move between retail, office, gym and clinic uses does not apply to it in the way it applies to a self-contained commercial unit. Anyone considering splitting one unit into two premises should work out the unit position first, because the split can create or destroy that flexibility.

Evidencing the position

Whether you are asserting one unit or two, the evidence is the same kind of material.

1. The planning history: every permission, the approved drawings, and any certificate of lawfulness already granted.
2. Tenancy agreements and leases, with the demise plans, over as long a period as you can reach.
3. Rating and council tax records showing how each part has been assessed.
4. Utility accounts showing separate or shared supplies, with meter positions.
5. Floor plans at the relevant dates showing internal connections, sealed openings and access arrangements.
6. Statutory declarations from occupiers, agents or managers describing how the parts were used and by whom.

Where the position matters commercially, an application for a certificate of lawfulness under section 191 of the 1990 Act is the way to settle it formally. The burden is on you and the standard is the balance of probability, so a chronology assembled before you apply is worth more than a persuasive covering letter.

Where you are applying for subdivision rather than confirming it, a Planning Statement should state the existing planning unit and the proposed units explicitly, with drawings showing the access and service arrangement for each. Officers deal with these applications more quickly when they do not have to work the unit analysis out for themselves.

And if a building is being bought on the assumption of two units, get the analysis done before exchange. The alternative is discovering that the flat you intended to extend has no Part 1 rights and sits in somebody else's curtilage.

Scoping the documents a subdivision application needs is a sensible first step once the unit position is clear.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A Lawful Development Certificate is the only formal way to confirm that works or uses are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.