We have a couple of great GC's that have done this kind of work before. I'll shoot some suggestions to you in an email. It can be fairly intensive and you wan to make sure the contractor includes procuring a 2-unit C of O (Certificate of Occupancy) and separately metering the new unit as part of the scope of work, so you can rent it out legally. You also want to make sure they build it out for maximum rentability (floorplan/finishes, etc).
Washington Property Management