Converting duplex into triplex

[Freddy German]

I own an RM-1 property in Philadelphia that is currently being used as a duplex. The basement is huge and unfinished, has an entrance that leads to it from the front porch and it’s own exit into the backyard. To me it seems like it would be the ideal space for another unit. Other properties on the same street have been converted into triplexes. Can anyone recommend a design build company that would handle any zoning needs with the city, as well as complete the construction and leave it rent-ready for me? Any advice would be greatly appreciated.