Aubrey, this should not be an issue since a duplex is a "higher use" typically city zoning will have no issue with approving a modification to a single family dwelling. In fact check the current zoning, quite often in the past a downtown Charleston a single famliy dwelling was converted to a duplex without a zonng change. In any event make sure you contact zoning to insure your pla is acceptable. Let me know if I can help in any way.
I do a lot of work downtown. Call me at 843-557-9203, or visit http://www.carolinajoe.com