To clarify, the property has to fulfil all three criteria (number of people / floors / households) to be classed as a House of Multiple Occupancy.
The Shelter website sums it up nicely in one sentence:
"The mandatory licensing scheme applies to larger houses in multiple occupation (HMOs), which the Housing Act 2004 defines as properties over three storeys with five or more occupants making up two or more households."
"The new rules close a common loophole used by landlords keen to avoid licensing: that students in shared accommodation should not be considered as a single household. An important practical change in the rules is that persons are to be regarded as not forming a single household unless they are all members of the same family."
Unfortunately the last sentence is not entirely correct as carers and domestic staff are classed as being part of the household but may not be part of the family.
Jackie will certainly have a locus to ask either Southwark or the building manageemnt about those flats which appear to be home to five or more students.
One issue about letting to single sharers is that you can usually count on double that number staying over on a occasional or sometimes near permanent basis. I never give permission for anyone to live in one of my properties unless they are on the tenancy agreement, and if I learn that someone has effectively moved in I object, but cannot and would not want to do anything about short stay guests. And often don't know.
Where do landlords stand on this?
(Ideally I would extend a couple of 2 storey properties into the attic, which would make them three storey, four resident properties.)
Philip RX8 is not entirely correct in all he writes. He correctly picks up that it is more complicated an issue than most councils or sources of advice appear to realise.
HMOs were in fact first defined in the 1985/1988 Housing Acts. Included within the definition are most properties with more than one household. Two unrelated, non-cohabiting individuals constitute two households.
The HA 2004 introduced mandatory licencing for all properties in multiple occupation of three of more stories, and five or more people.
The consequence of this is the landlord must instal inter alia firedoors and mains fire alarms.
Individual councils are free to introduce these requirements (fire precautions, licencing) for lower levels of occupation. Westminster being Westminster virtually asks you to put fire alarms in if you are an OAP living on your own and have a CAT living with you with whom (which?) you are not sexually involved. Southwark being Southwark is pretty relaxed, even with five or more people in properties of three or more stories. Southwark has bigger problems with its housing to worry too much about this sort of thing.