I am going through the process now. I hired an architect to draw me up plans to submit to DCRA. Currently I am in the process of responding to DCRA’s comments. Now I am looking for contractors who can build my plans. Let me know if you have any leads! I subscribe to Angie’s List, but I generally find it to be useless.
A correction to what odcstudio wrote above: minimum ceiling heights for habitable spaces in DC are 7’0″ generally (with limited exceptions for lower heights near beams, bathrooms, closets, etc), not 7’6″. See DC Municipal Regulations 14-405.