I don't know what the rules are in SF, but in NY, you are only allowed to hand a rent-controlled apartment down once. After that, the apartment can be raised to market value, or evicted (something like that). I don't know a ton about the NY rent laws, but we ran into a problem like this when were living in Queens. The guy who bought our building tried some very illegal tactics to scare people (including us) out of our apartments. That's a risk you have to take as a property buyer; you can't just buy a lived-in space and be allowed to kick people out who lawfully have a lease. You either have to wait them out, hope they leave on their own, or be really sneaky and try to catch someone who isn't following the regulations of their lease.Does the apartment owner owe them well-below-market housing? Until they die? What if that never happens, they marry someone younger, and die, and that person marries someone younger, then dies, keeping the same lease active for decades?