Legally creating new "units" vs. leasing rooms ad hoc

[Alexander V.]

A SFH is different from a duplex, which is different from a triplex and 4plex, which are all totally different from 5+ units. But legally speaking, what makes something a "unit" in the view of the city planners as opposed to an ad hoc division on the property? For example, my cousin in college rented a single room in a 4 bedroom house. It was a SFH, but it had 4 different people renting 4 different rooms. I've heard of this particularly in college towns and in areas like SF where costs are extremely high. Is there any legal barrier to leasing subdivided spaces in a large house on an ad hoc basis as opposed to partitioning them off as "units" with individual meters and entrances?

I'm just trying to wrap my head around zoning laws. It seems like a case of unnecessary complication--if you lease out 3 separate rooms, it's fine. If you label them as "units" and add separate meters, suddenly you have a billion legal papers to file, city officials to navigate, and rezoning fees to pay.