Turning a 2 unit into a 3 unit

[Bennett Steinle]

I recently looked at a 2 unit apartment however I believe with a bit of work it could be converted into a three unit. 

Are there any legal things I need to worry about with zoning and things like that? I know every area is different so I might not be able to get a full answer here. I'm just not sure exactly who I need to call to or what goes into a process like this so any insight would be valuable.