Luke you are correct about the mandatory licensing but not quite correct about the definition of an HMO.
A small HMO (planning use class C4) is:
- C4 Houses in multiple occupation - small shared houses occupied by between three and six unrelated individuals, as their only or main residence, who share basic amenities such as a kitchen or bathroom.
A larger HMO has nothing to do with the number of floors just that it contains 7 or more tenants. A large HMO would fall into the "Sui Generis" planning use class.
See this link for more information:
http://www.planningportal.gov.uk/permission/commonprojects/changeofuse The number of floors simply relates to licensing (in most boroughs I'm operating in and aware of anyway) and has nothing to do with the planning definitions...
So
@KGeeson your situation would be a "C4" small HMO.
But don't worry! It is highly unlikely to be licenseable (check your councils website to be sure) and it is permitted development to move from C3 (single household) to C4 small HMO. So you are not in breech of any planning as long as it's been used as C3 for a reasonable amount of time before going to C4 use (6 months is normally deemed not de-minimus).