Legality of converting Garage to Unit?

[Tim Kaminski]

Looking at a 2 building duplex that has a garage on first floor of the second unit. MLS said the previous owner had converted the garage into a 3rd unit for a time and it was able to be easily converted back to a 3rd unit. I'm curious if I would be able to do that legally, and if so, how? The property tax website has it listed as a multiple unit property, but just showing the 2 total units. Walking the unit Tuesday so can give more info then, but would like some knowledge going into it if I should be looking at it like a potential triplex.