The California Landlord's Law Book: Rights and Responsibilities, Chapter 10, "Cotenants, Subtenants, and Guests" includes a sidebar on the "master tenant" arrangement that's possible under the San Francisco rent control ordinance. The sidebar explains that tenancies with master tenants, which are common in San Francisco, occur when an original group of cotenants loses members but at least one original tenant stays. That remaining tenant, known as a master tenant, often brings in a series of new roommates, who are subtenants. The sidebar concludes with the suggestion that landlords should make late-arriving subtenants full-fledged cotenants, to remove a layer of management and make every tenant equally answerable to the landlord.

The sidebar's concluding advice is correct for landlords who are not subject to San Francisco's rent control ordinance. But in San Francisco, due to a provision in the rent control ordinance that limits the situations in which landlords may raise the rent to market levels, this advice is not advantageous for those landlords whose properties are subject to the ordinance.

Under San Francisco's rent control ordinance, landlords are prohibited from raising the rent to market levels as long as an original tenant -- the master tenant -- still remains. If the landlord adds a new occupant as a cotenant, and the original master tenant leaves before that new tenant does, the landlord will not be able to raise the rent to market levels. For this reason, it's to the landlord's advantage to treat new occupants as subtenants, and not make them into cotenants. Then, if the master tenant leaves, the landlord can raise the rent for the remaining subtenants to market rates, because the later arriving tenant is a subtenant, not a cotenant.

San Francisco landlords who want to preserve their right to raise the rent to market levels when the master tenant leaves must explain this part of the rent control ordinance to subtenants shortly after they move in, by giving them a "6.14" notice (named after the ordinance section).