In Calif. if a landlord HAS allowed pets to an individual tenant, s/he cannot then DISallow them for a particular owner when s/he changes her/his mind.
My mom really benefited from that because she is an excellent steward of cats-- catch and release for spays, and vet care for the diseased. Not a cat hoarder. The landlord got pissed off one day to see kitteh food in a bowl on the back step, and tried to ban her from "keeping" any critters. "Yer attracting raccoons!" "I'm luring cats to be spayed to keep the "coons DOWN," she replied, and then he said she'd have to move. She quoted chapter and verse right back to THAT and that was the end of THAT!
At lease renewal time he also cannot fail to renew based on pet issues because he already allowed them.
Scary-- so much common sense in Calif.!
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As to gnu's point about future allergies, it's pretty simple. It's just like hotels and smokers. A room posted as "no smoking" does not mean there has NEVER been smoke in there, but it does mean that smoke has been removed to the extent possible. Thus, it's not a guarantee, but a designated zone within which a person with allergies can take their own measures to deal with whatever is the case upon entry.