Quick Answer

C3 to C4: The Use Class Change

Under the Town and Country Planning (Use Classes) Order 1987 (as amended), Class C3 covers a house occupied by a single family or up to 6 people living as a single household. Class C4 covers a small HMO — a house occupied by 3 to 6 unrelated people who share facilities but are not a single household. Sui generis covers HMOs with 7 or more occupiers, which always require planning permission for any change of use.

When It’s Permitted Development

The change of use from C3 (dwellinghouse) to C4 (small HMO) and back is permitted development under Part 3, Class L of the GPDO 2015. Where no Article 4 Direction has removed the right, you can convert a house to a small HMO without any planning application.

Article 4 Directions for HMOs

Many councils in cities, university towns, and areas with high HMO concentrations have made Article 4 Directions removing the C3-to-C4 permitted development right. Where an Article 4 Direction applies, planning permission is required. The council will assess the application against its HMO policy — typically a threshold-based approach that refuses where HMOs already exceed a certain percentage in the immediate area.

Large HMOs: Sui Generis

An HMO with 7 or more occupiers falls outside the use classes — it is sui generis and always requires planning permission. Converting a C4 small HMO to a large HMO (adding a 7th bedroom) also constitutes development requiring planning permission — it’s a change from C4 to sui generis.

HMO Licensing

HMO licensing is separate from planning and applies regardless of whether planning permission was needed. Mandatory HMO licences are required for HMOs with 5 or more occupiers in buildings of 3 or more storeys. Some councils have additional licensing schemes covering smaller HMOs. Operating an unlicensed HMO is a criminal offence, and tenants can apply for a rent repayment order to reclaim up to 12 months’ rent.