Renting permitted addition that is not ADU

[Lindsay Bowman]

My offer just got accepted on a 3 unit place. After looking closer I realize there’s only 2 addresses and Redfin has it listed as a single family. Im told all units are permitted and the “unit” that doesn’t have an address is attached to the original construction. My question is, is it legal to rent that as a unit even though it doesn’t have its own address? Im assuming it’s permitted as an addition not as an adu or else it would have an address right? 

Thank you in advance!