Ancillary use: when a subordinate use becomes a new one

An ancillary use is an activity that serves the main use of a site and has no independent existence of its own. It does not have its own use class, it is not assessed separately, and it needs no permission while it stays subordinate. The moment it stops serving the main use and starts functioning on its own terms, a material change of use has occurred under section 55 of the Town and Country Planning Act 1990, and that is development requiring planning permission.

For a landlord this is not an academic point. It decides whether the flat over the shop needs an application, whether the annexe has become a dwelling, and whether an enforcement officer has a case.

What makes a use ancillary

Three characteristics run through every ancillary use.

It serves the primary use rather than the wider public. A staff canteen serves the factory. A storeroom serves the shop. An office in a warehouse serves the warehouse operation.

It is subordinate in scale. Floor area, hours of operation, staff numbers and the proportion of activity all feed into this, and none of them is decisive on its own.

It shares the occupation and control of the primary use. Where an activity is let separately, run by a different operator or occupied independently, the argument that it remains ancillary becomes very hard to sustain.

Ancillary status attaches to the facts on the ground rather than to the intention. A room described as ancillary storage in a lease is ancillary storage only for as long as that is what it is used for.

Ancillary compared with incidental

The two words are used loosely in conversation and precisely in planning.

Incidental has a statutory home. Section 55(2)(d) provides that the use of buildings or other land within the curtilage of a dwellinghouse for any purpose incidental to the enjoyment of the dwellinghouse as such is not development. That is the provision behind home gyms, garden offices, workshops and hobby rooms, and it is limited to residential curtilage.

Ancillary is the broader concept and applies to any primary use, residential or commercial. It comes from case law on the planning unit rather than from a statutory definition.

The practical difference is scope. The incidental test asks whether the activity is a normal part of enjoying a house. The ancillary test asks whether the activity serves the main use of the planning unit. A large storage building in a garden may be incidental. The same building used to store stock for an online business is not, because it serves a trading operation rather than the enjoyment of the house.

The single planning unit concept

Everything about ancillary use depends on identifying the planning unit, because the unit is what gets assessed.

The approach the courts take starts with the unit of occupation. Where the whole of that unit is used for a single main purpose, with activities that serve that purpose, the whole is one planning unit and the subordinate activities are ancillary. Where physically separate areas within the occupation are used for substantially different and unrelated purposes, each of those areas can be a separate planning unit in its own right.

That is why a shop with a flat above, both occupied by the shopkeeper, can be one unit, while the same building with a separately let flat is two. The detail of how councils decide what they assess matters here, because the boundary of the unit decides which changes are material.

The unit can change. Dividing a building, letting part of it separately, or blocking an internal connection can create a new planning unit without any building work that needs permission in itself.

The point at which a separate use arises

There is no threshold in the legislation. What exists is a set of indicators, and officers weigh them together.

Independent access. Separate services and metering. Its own entrance, postal address or council tax band. Separate occupation under a different agreement. Its own trading hours and customers. Physical separation from the primary use. A scale that no longer looks subordinate. And the loss of any functional relationship with the main activity.

Intensification alone can be enough. An activity that was plainly ancillary at a small scale can become the dominant use without anything else changing, and the change is material at the point where the character of the whole unit has altered.

Officers usually approach it by asking what a visitor would say the premises are used for. If the honest answer names two things rather than one, the position has probably moved.

Annexes, home working and short lets

Three situations produce most of the disputes.

An annexe stays ancillary to the main house while it is occupied as part of that household, with shared facilities and no independent existence. Once it has its own kitchen, its own entrance, its own council tax banding and an unconnected occupier paying rent, it is a separate dwelling and the creation of that dwelling needed permission. The ancillary occupation test for annexes turns on occupation rather than on the layout alone.

Home working is generally incidental until it changes the character of the property. Visitors arriving, staff attending, deliveries in commercial quantities, signage, vehicle movements and noise are what move it across the line. One person at a laptop does not.

Short letting is different again, and in London it has a statutory limit. Under the Greater London Council (General Powers) Act 1973 as amended by the Deregulation Act 2015, providing temporary sleeping accommodation in a London residential property for more than 90 nights in a calendar year is a material change of use requiring planning permission. Outside London there is no equivalent statutory limit, and the question falls back on whether the character of the use has changed.

Enforcement consequences of use drift

Use drift is gradual, which is what makes it dangerous. Nobody decides to create an unauthorised use.

Enforcement against a material change of use is time limited, and the limits in England were extended by the Levelling-up and Regeneration Act 2023, so check the position that applies to your circumstances and the date the use began. Immunity is not the same as lawfulness until it is certified.

The consequences of getting it wrong run beyond enforcement. An unauthorised use undermines a sale, because a buyer's solicitor will raise it and a lender may decline. It can invalidate insurance. And where a building has been divided into units without permission, council tax and licensing positions often expose the planning position before any officer visits.

Retail premises with residential above are a frequent example, and the route from retail to residential above a shop is worth following properly rather than allowing the use to drift into existence.

Evidencing the position with records

If you are relying on a use being ancillary, keep the evidence that shows it. If you are relying on a use being lawful through the passage of time, keep the evidence that shows when it started and that it has continued.

Useful records include tenancy agreements and licences, council tax and business rates records, utility accounts and meter arrangements, floor plans showing internal connections, dated photographs, trading records showing hours and turnover by part of the premises, and statements from people with direct knowledge.

The formal route to certainty is a lawful development certificate under section 191 for an existing use or section 192 for a proposed one. That is the only way to confirm that a use is lawful, and an application supported by a statutory declaration and contemporaneous records is far stronger than one supported by recollection.

Where the boundary is uncertain, apply. A refused certificate tells you where you stand at a fraction of the cost of an enforcement notice. Browse the full list of report types to see what supporting evidence an application of that kind usually needs.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance rather than legal advice, and you should check the position with your local planning authority or a qualified professional before relying on it.