Adding 2 unit addition to 2 unit house

[Jake August]

Hey everyone!

So I’m at a fork in the road where I have a two unit apartment with great numbers and a big lot on the side along with space in the back. I could buy another house but I think this would pay for itself faster. The house already has 3 bedrooms on each floor and is located in an improving area. If i were to add another two units what would be my first step to see if I can legally do this without spending too much money? I can do most of the work myself except for the slab/foundation. Sorry in advance if these are stupid questions but if anyone can point me in the right direction that would be great! Thanks again