Downstair unit question

[Wen Chuang]

Dear all,

A relative of mine recently purchased a single family home in SF, and intend to remodel and rent it out as a single family home (no APU, no rent control). There is a bonus room downstairs in the garage (not sure if it's warranted). The room does not seem to be inhabitable, and we think it's not suitable to be used as a bedroom (it is right next to parked cars). He is worried IF the future tenant use that room for AirBnB purposes or a guest room, and if something happened, whether he (as a landlord) would have any liability for that. Any thoughts or comments on this?

If he specifically add this policy (no AirBnB or use downstairs bonus room as a bedroom) in the lease, would it be enforceable? 

Thanks!