A house in multiple occupation (HMO) is a building, or part of a building, that is being lived in by a group of people.
For example:
A house or flat converted into three or more bedsits
A house or flat occupied by three or more friends, work colleagues or students
A house that has self-contained flats where less than two thirds of the flats are owner-occupied. The flats were not converted in accordance with the 1991 Building Regulations