Interesting. Amsterdam actually has a scheme that promotes this ('woongroep') for the express purpose of accommodating students.

In America, all it takes is a developer not making their money back (i.e. not wanting to be a “risk-taker” anymore) and these sorts of laws magically disappear.

Indeed, however, the number is usually very reasonable, usually 5+, its not gonna stop a grad student from saving money on rent.

Though the court case that upheld these laws as constitutional, Village of Belle Terre v. Boraas, the zoning law was extreme at limiting only two unrelated persons - which is extreme.

Reasonable to you, but others may think 20+ is fine. I’m not sure what the law is even trying to protect, or why it matters.

It's zoning ordinances and like any zoning ordinance its aimed at controlling or maintaining the character of the neighborhood, preventing frat houses and group homes in certain neighborhoods, etc. If parking for one family is expected and 8 students take up the whole block, etc. It's limited to unrelated persons because "family status" is a protected class under housing law.

Different houses on the same street can be zoned differently. My house is zoned "single family" but two houses down is zoned "boarding house."