Building a modern extension to an existing museum, church or other historic site can be problematic. These buildings are often situated in old and overpopulated cities. 'C3' takes a closer look at how today's architects have solved this dilemma. The Dresden Military History Museum by Daniel Libeskind, The new Messner Mountain Museum by EM2 Architects and the restoration of Madrid’s Slaughterhouse by Pesquera Ulargui Arquitectos serve as key examples. Other projects featured are: Milstein Hall at Cornell University by OMA, the Drents Museum extension by Erick van Egeraat Architects, and the Theater of Châtelard by Clermont Architects.