I don't think that is true, or at least it doesn't have to be true. First, imagine a world where instead of "you need four parking spots if you want a duplex," the law was more flexible and said something like "the # of cars registered to an address cannot be greater than the # of parking spots at that address." That would solve the problem but also allow the duplex owner to rent to at least some subset of tenants instead of not being able to rent at all. Second, my understanding is older houses are grandfathered in, so if I owned a duplex built 50 years ago that wouldn't pass duplex code today, I would still be able to use it as a duplex without bringing it up to code. That means there are already duplexes with two parking spots that are legal, but I just can't have one if I build it today with more modern, safer building methods. (IANA real estate laywer, but this grandfathering is my understanding of how zoning law works.)
> So a property not designed to hold a large number of people, isn't allowed to hold a large number of people. That seems reasonable. Just from a fire-safety perspective alone it makes a lot of sense to require custom-built commercial facilities after a certain number of kids.
I understand the need for some requirements, like fire safety. That's very important and there are separate rules for that. But California requires something like 35 indoor sq feet per child in a home day care facility. That means you can have 14 kids in a 1,000 square foot home, but can't have 15 kids in a 3,000 square foot home. These laws are very blunt instruments.