To get into a building you need an access card/fob or to use the intercom.
If you have extra tenants then you need them to be able to get into the unit thus you give them an access card/fob, but it’s limited, 2 bedroom = 4 access card/fob etc..
So what the slumlord will do if he has 6 extra tenants?
He will buy or make himself duplicated access card/fob for his extra tenants or he will install a machine called Black Box into the unit intercom receiver so the receiver can open the lobby door and lift level without having to physically press the button Open.
If the proper security access control is installed then you have all extra tenants in big difficulties to get into the units.
They won’t stay.