Sui generis

Sui generis is a planning term meaning “in a class of its own” in particular for HMOs. In England, it describes uses of land or buildings that do not fall within a standard use class under the Town and Country Planning (Use Classes) Order, so they are treated separately for change of use decisions. The Planning Portal explains that use classes group similar uses, while some uses are treated differently outside the classes. 

For most landlords, sui generis comes up when a residential property is or could become a large HMO. Many councils describe “small HMOs” as Class C4 (3–6 unrelated occupants sharing facilities) and “large HMOs” as sui generis (typically 7 or more occupants). If your rental property moves into a sui generis use, you will usually need planning permission for the change of use. This is separate from licensing under housing law: you can need one, the other, or both.

Getting the planning status wrong can lead to enforcement action by the local housing authority, difficulty refinancing or selling, and operational headaches. For example, if you have signed a tenancy agreement but cannot lawfully use the building as intended. It can also affect building standards expectations (fire safety layouts, waste storage, noise management) that link directly to property condition, repairs, and future housing disputes.

Also watch out for local restrictions. Some areas use an Article 4 direction to remove permitted development rights, meaning even a move from a normal dwelling to a small HMO can require planning consent. Your local council’s specific planning pages will confirm whether this applies.

The Renters’ Rights Act doesn’t change planning law, but it changes your tenancy risk profile. Section 21 notice ends and most lettings move to an open-ended periodic tenancy, so any “wrong use” problem that disrupts occupation is harder to solve by simply ending the tenancy. Plan the use class, consents and paperwork before you let, and keep everything in written notice form.

Also see our landlord blog articles, including:

• What is an HMO and when do I need a licence?

• Is your property actually an HMO? You might be surprised

• Understanding landlord licensing across England and Wales

• Landlord legislation outlined across the UK and how they differ by region

• 14 new rules to prepare for as a landlord in 2025

• Your comprehensive guide to UK property management

• How to become a successful student accommodation landlord

• Landlord licensing in 2026: Which LHAs are adding new schemes

• The next HMO evolution for UK landlords

Also see our free landlord calculators:

• HMO Calculator