This thesis explores how to design an addition to a modern building that makes the building users cognizant of the layers of history while simultaneously creating spaces that respond to the needs of the occupants today. The existing building, the Washington D.C. Central Library, designed by Mies van der Rohe, is conserved and rehabilitated maintaining the library function and new space is created for the Foundation for the Study of Social Media. The addition and the surgical rehabilitation of the existing are informed by the design concepts and details of Mies van der Rohe as well as the theories of Carlo Scarpa. The project evolves through a detail outward design approach in which material and spatial relationships at critical joints are first determined and then used to inform the design of the whole.