Severance of a planning unit and what it changes

Severance of a planning unit means splitting one unit of occupation into two or more, usually by selling or letting part of it. The reason it matters to anyone running a business from a building is that severance can turn a lawful ancillary use into an unauthorised primary use overnight, without a brick being moved. The activity carries on exactly as before. Its planning character changes because the unit around it has changed.

For a dwelling the position is settled by statute: use as two or more separate dwellings is a material change of use. For commercial premises the answer depends on what each part is then used for, and on whether that use differs in character from the use of the whole.

What a planning unit is

The planning unit is the parcel of land against which you judge whether a material change of use has happened. It is not the red line on an application form and it is not a title boundary, although both can be evidence of it.

The working test comes from Burdle, decided in 1977, and it has three limbs. Where an occupier carries on a variety of activities across their whole holding and none can be described as incidental to another, the planning unit is the whole unit of occupation. Where two or more physically separate and distinct areas within one occupation are used for substantially different and unrelated purposes, each area is its own planning unit. And where there is one primary use with other activities incidental to it, the unit of occupation is the planning unit and the incidental activities do not form units of their own.

That framework does most of the work in practice, and the question of what the planning unit is in any given case has to be answered before you can say anything useful about change of use.

Primary and ancillary uses inside one unit

Inside a single planning unit, an ancillary use needs no permission of its own. A storage room behind a shop, a small office above a workshop, a staff kitchen in a gym, a paddock used with a dwelling: these ride on the primary use and have no independent planning status.

Two characteristics make a use ancillary. It serves the primary use rather than the public or a separate market, and it is subordinate in scale and character. A shop with a back room storing stock is one use. The same shop with a back room selling tyres to trade customers through a separate door is arguably two.

The important point for what follows is that ancillary status is a relationship, not a property of the activity. Break the relationship and the activity has to stand on its own. Where an ancillary use ends and a new use begins is the line that severance walks straight across.

What severance means in practice

Severance happens in ordinary commercial ways, and most of them look like conveyancing rather than planning:

• Selling the yard, the upper floors or a rear building separately from the front unit.
• Granting a lease of part of the premises to an unconnected occupier.
• Splitting a large retail unit into two lettable shops.
• A family selling the field behind the farmhouse to a neighbour.
• A pub selling off its function room as a separate business.

In each case the single unit of occupation becomes two. The physical fabric may be untouched, and often the first sign of a problem is a planning enforcement enquiry two years later, prompted by a licensing application or a business rates entry.

Whether severance is itself development

Severance by itself is not development. Dividing land into two ownerships or two tenancies is not an operation and is not, on its own, a change of use.

What severance does is change the unit against which use is measured. After the split, each part has to be assessed on its own. Where the use of a part, judged alone, is materially different in character from the use of the whole before the split, there has been a material change of use and permission is needed. Where it is not, nothing has happened in planning terms.

Two examples make the distinction clear. A large shop split into two shops of the same character is usually no change of use at all, because each part retains the use the whole had. The same shop split so the rear becomes a self-contained storage and distribution operation serving other businesses is a change, because the rear part, considered alone, is no longer retail.

The statutory exception sits in section 55(3)(a) of the Town and Country Planning Act 1990: the use as two or more separate dwellings of a building previously used as a single dwelling is a material change of use of the building and of each part so used. There is no argument to be had about character there. The general test for a material change of use still governs everything outside that provision.

Subdividing a shop, and subdividing a house

The two cases behave differently and it is worth setting them side by side.

For the commercial case, the practical questions an officer asks are about servicing, refuse, access, parking and hours, because those are the effects that make a subdivided premises behave differently from the single unit. A subdivision that creates two units each needing a delivery bay where one existed before will attract a highways objection whether or not the use class has changed.

What severance does to later change of use rights

This is where severance costs money, and it does so in three ways.

Prior approval routes have qualifying conditions attached to a building and its use over a period. Where those conditions are expressed by reference to the building having been in a particular use for a stated time, a severance that changed the character of part of the building can break the qualification. Check the condition wording against the history rather than against the current arrangement.

Permitted development rights attach to the planning unit. A right to extend, to install plant, or to change use applies to a unit, so after severance each new unit has its own allowance and its own history of previous development. That can help, by giving two allowances where there was one, or hurt, where the earlier development has used up the allowance on the part you now own.

Lawful use evidence fragments. If you later need a certificate under section 191 to establish the lawful use of your part, you have to prove the use of that part, not of the original whole. Documents held by a previous owner of the other half may be the only evidence, and they are rarely available.

Where the commercial plan involves splitting premises and then converting part, sequence the applications deliberately. Establishing the position on the whole first, then severing, is usually safer than severing and arguing later.

Evidence that fixes the unit boundary

Officers and inspectors look at how the land is actually occupied and used, and the documentary record is what you have to work with years afterwards. Keep these:

1. Leases and transfers, with plans, dated.
2. Business rates entries for each part, which are the single most commonly cited external indicator.
3. Utility meter arrangements and any separation works.
4. Photographs of physical division: walls, doors sealed, separate entrances, boundary treatments.
5. Staff arrangements, separate tills or booking systems, separate insurance and separate accounts.
6. Any planning history for either part, including certificates and enforcement correspondence.

None of these is decisive on its own. Together they establish a picture, and the absence of all of them is why so many severance disputes turn into arguments about a memory.

Where you are applying for anything after a severance, describe the unit in the Planning Statement rather than assuming the officer will infer it from the drawings. State what the unit was, when it was split, what each part has been used for since, and what the application unit is now. PlanScope drafts that statement from the answers you give and flags the facts it cannot confirm.

A lawful development certificate is the only formal way to confirm that a use is lawful, and on a severed unit it is often worth the fee before a sale or a lease. Enter your postcode on PlanScope to see which documents a change of use or subdivision is likely to need.

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.