What to consider when building a multi-unit property??

[Nhi Tran]

Hi Bigger Pockets members! I have an investment idea that I'm hoping the experienced members can help me analyze in terms what I need to consider and the how-to's. 

My husband I own two duplexes across from each other. Each duplex is 1,560 square feet and each is on a 10,000 sq feet lot of land. Each unit brings in $585/month. We're considering building another duplex (or perhaps even a two-story four plex) on each lot. We have enough cash on hands to build 2 additional multiplex units and do not need to take out any loan. 

With the new builds, I'm hoping to not spend more than $30K-$35K on a duplex and $60-$70K on a fourplex. I should also mention that the location is not the best. It is in a little rougher neighborhood of town. I think we should be able to get around $600/unit with the new build. 

My questions are: what do we need to consider when thinking about building the multi-units? would finding a general contract suffice? Would the GC take care of all the permitting for us or is this something we're responsible for? W'ere also not really familiar with costs - what kind of expenses do we need to consider?  Any inputs are appreciated! Thank you!