Seeking Advice : Converted Garage Property

[Prachi Metha]

I am looking into few house where with garage is either converted into 1bed/1bath house or either have some 1 additional room in backyard. What are things I have to be careful about while dealing with these property? None of these property seems to have approval from city seems like, can someone guide me on process it take to get approval from the city? 

Experts Please Advice. Thank you for advice in advance.