Houses in Multiple Occupation (HMO) License
An HMO must have a licence if it is occupied by 5 or more people. A council can also include other types of HMOs for licensing.
Your property is an HMO if both of the following apply:
at least 3 tenants live there, forming more than one household
toilet, bathroom or kitchen facilities are shared
A household consists of either a single person or members of the same family who live together. It includes people who are married or living together and people in same-sex relationships.
The council has to carry out a Housing Health and Safety Rating System (HHSRS) risk assessment on your HMO within 5 years of receiving a licence application. If the inspector finds any unacceptable risks during the assessment, you must carry out work to eliminate them.
(ref: https://www.gov.uk/renting-out-a-property/houses-in-multiple-occupation-hmo )
for more info: