In the UK rental market, HMO stands for 'house in multiple occupation' (or sometimes 'house of multiple occupancy'). UK law defines a rental property as an HMO if both of the following apply:
At least three tenants live there, forming more than one household
Toilet, bathroom or kitchen facilities are shared
A ‘household’ means either a single person or members of the same family who live together and this includes couples who are married or living together and people in same-sex relationships.
So, a property would be classed as an HMO if it is shared by two or more individuals or groups that are not family members, a married couple or in a relationship. A common term to describe this kind of arrangement is a ‘house share’.