A single adult with no children can rent a room in a house or apartment with other friends. A couple can do this too, but some of them prefer to have their own

A single adult with no children can rent a room in a house or apartment with other friends. A couple can do this too, but some of them prefer to have their own place so this skews the average a bit. Once you have a kid it's a lot harder to find people who want to live with you so you have to get your own place. This place is apparently big enough to support up to two adults and two children after which you need a bigger place.