Including 5th unit in a fourplex

[Raj Janakarajan]

Hello, I am out of state investor and recently bought a fourplex with a large backyard (Lot size 10k sq ft). There is enough space to include a separate unit in the backyard.  I am looking for advise on steps I can take to find out if it is possible to include a 5th unit in the backyard.  There is already a structure there that was used for storage I believe with foundation.  Does anybody know if it is even possible to do that?  

Raj