Recently a couple of buyers have asked about the "NY Roommate Law" pertaining to "bylaws" rules and policies in a coop. Last year while I attended an exhibit at the Museum of New York about small spaces and the trend around the world of living alone, it was at that exhibit that I first learned it is illegal in NY for more than three unrelated people to live in an apartment or a house.
The law, for decades part of the city’s Housing Maintenance Code, is little known, widely broken and infrequently enforced. NY has many housing laws that were reforms from the 19th and early 20th century tenements.
A landlord or coop may restrict the number of occupants based on housing/occupancy laws but you can generally take on roommates without having to add them to the lease or even get your landlord's approval.
Recently I've learned of "NY Roommate Law" Unlawful Restrictions on Occupancy Law. A law that entitles a tenant to have a roommate. This law trumps coop "bylaws" regarding the coop's right to approve all occupants living in a coop apartment.
Roommate Law Explanation:
If you are the only person who has signed the lease, in addition to your immediate family, NY law allows you to have another roommate who has not signed the lease. The roommate’s dependent children are also permitted. (Note: This law applies if the premises is the tenant’s primary residence)
This law allows tenants with leases to have roommates without interference from their landlords. While many tenant protection laws do not apply to cooperative apartments, the so-called Roommate Law does.
Real Property Law, Section 235f "Roommate Law"