Sui generis

Sui generis is a Latin phrase meaning "of its own kind". In English planning law it describes a use of land or buildings that does not fit within any of the standard use classes set out in the Town and Country Planning (Use Classes) Order 1987. Because sui generis uses have no class, they have no permitted development rights, any change to or from a sui generis use always requires a formal planning application. For landlords, the term is most relevant in the context of large HMOs: a property housing seven or more unrelated people sharing facilities is treated as sui generis rather than falling within a standard residential class.

The planning use class hierarchy for HMOs

The residential planning use classes that matter for landlords in England are:

Class C3 (dwellinghouse). A single household, a family, a couple, or a person living alone. Properties sharing by groups that are not HMOs under the Housing Act 2004 also fall here.

Class C4 (small HMO). A property shared by three to six unrelated people as their only or principal home. Class C4 was introduced in April 2010 specifically to allow planning authorities to regulate the proliferation of smaller shared houses. A change from C3 to C4 is ordinarily permitted development, meaning no planning application is needed, unless the local authority has removed that right using an Article 4 direction.

Sui generis (large HMO). Any HMO with seven or more occupants falls outside Class C4 and is classified as sui generis. A move from any use class, C3, C4, or any other, into a sui generis large HMO always requires full planning permission. There is no permitted development route and no Article 4 direction is needed to trigger the requirement; it applies everywhere in England as a matter of law.

When planning permission is required

The planning permission trigger points for HMO conversions are:

A change from C3 to C4 (three to six occupants) does not normally require planning permission, unless the property is within an Article 4 direction area. Where Article 4 applies, even this smaller conversion requires a full application.

A change from C3 or C4 to sui generis (seven or more occupants) always requires full planning permission, everywhere in England, regardless of Article 4 directions. The local planning authority will assess the application on its individual merits, including impact on the local area, parking, amenity, and the concentration of HMOs nearby.

A change from one sui generis use to another, for example, converting a nightclub to a large HMO, also always requires full planning permission.

Planning classification as sui generis does not itself trigger mandatory HMO licensing, these are two separate legal regimes under different legislation, but a large HMO will typically require both: full planning permission under the Town and Country Planning Act and a mandatory HMO licence under Part 2 of the Housing Act 2004. August's compliance checklist covers both HMO licence status and the safety certificates that apply to large HMOs simultaneously.

Article 4 directions

An Article 4 direction is a tool used by local planning authorities to remove a specific category of permitted development rights within a defined area. For HMOs, Article 4 is most commonly used to require planning permission for C3 to C4 conversions, moves that would otherwise be permitted development. This means that in Article 4 areas, even a small shared house of three to six people requires a planning application.

Article 4 does not affect sui generis uses, since large HMOs already require full planning permission everywhere. Its significance for landlords in Article 4 areas is that it makes the jump from a single dwelling to any HMO (not just a large one) subject to council scrutiny. The local planning authority's website will confirm whether an Article 4 direction is in force for a particular address.

Consequences of the wrong planning classification

Operating a property as a sui generis large HMO without planning permission is an enforcement risk with serious practical consequences. The local planning authority can serve an enforcement notice requiring the use to stop or the property to be returned to its lawful use. Failure to comply with an enforcement notice is a criminal offence. Planning enforcement can also make it difficult or impossible to refinance or sell the property, since lenders and buyers will expect a clear planning history.

Where a property has been operating as a large HMO without permission for a continuous period of ten years, it may be possible to apply for a Lawful Development Certificate. This is a formal document from the local planning authority confirming the use has been continuous and is lawful by efflux of time, it does not grant planning permission retrospectively but it regularises the position. For a period below ten years, a retrospective planning application for change of use is the usual route.

Getting planning status wrong can also have tenancy implications. From 1 May 2026 under the Renters' Rights Act, most tenancies are open-ended assured periodic tenancies and ending them requires statutory grounds for possession. A property caught in a planning enforcement situation is harder to manage than one on a clear legal footing, the landlord cannot simply end the tenancy to resolve a planning problem.

For a practical guide to identifying whether a property is an HMO, which licensing regime applies, and how the planning and housing definitions interact, see our article on whether your property is actually an HMO.

For a quick comparison of HMO yield against single-let yield, see the August HMO calculator.

Frequently asked questions

How many occupants make an HMO sui generis?

Seven or more unrelated occupants sharing facilities classifies a property as sui generis in planning terms. Properties shared by three to six unrelated people fall into Class C4. The threshold is based on occupancy numbers, not the physical structure. A property can be physically identical to a smaller HMO but require full planning permission simply because of the number of people using it.

Does a sui generis HMO always need planning permission?

Yes, without exception. Unlike Class C4 where permitted development rights may apply (subject to Article 4 directions), sui generis uses have no permitted development rights at all. Any change of use to or from a sui generis use, from any existing use class, requires a formal planning application to the local planning authority.

Is planning permission the same as an HMO licence?

No. They are entirely separate. Planning permission is granted by the local planning authority under the Town and Country Planning Act and governs the lawful use of the land. An HMO licence is issued by the local housing authority under the Housing Act 2004 and governs the management and safety standards of the HMO. A large HMO will usually need both, and failing to have either is a separate legal risk.

What is a Lawful Development Certificate for a large HMO?

A Lawful Development Certificate is a formal determination by the local planning authority that a use is lawful, either because it has been continuous for ten years (for sui generis uses) or because planning permission was properly granted. It does not retrospectively grant planning permission but it provides a definitive record of the use's legal status, which matters when refinancing, selling, or defending against enforcement. A property operating as a large HMO without permission for less than ten years cannot rely on this route and must instead seek retrospective planning consent.