Renting out a back house unit that isn't permitted

[Patrick McCandless]

Found an awesome 2 acre property that has a 1,500sf single family home with 2-car garage and then a separate garage/workshop with a 1 bedroom apartment on top. The seller currently lives in the 1br and VRBO's the main house. Initial thoughts are doing the opposite and live in the main house while renting the apartment either log term or VRBO/Air BnB but the apartment unit is not permitted per the listing agent. This is in Central California. I'm looking for some advice from landlords with experience in renting out back houses or mother-in-law units like this. Things I should take into consideration or watch out for etc. And also if it not being permitted is anything to worry about as long as it is still safe and perfectly habitable. Thanks in advance!