The chapter focuses on the significance and the originality of the study of the exaptation as a possibility to extend the architectural design toward more sustainable approaches aimed at enforcing urban resilience. The use of exaptation’s definition in architecture corroborates the heuristic value of the cross-disciplinary studies on biology and architecture, which seems even more relevant in times of global environmental crises. Exaptation will be described, in the chapter, as a functional shift of a structure that already had a prior but different function. In architecture, a functional shift of a structure that already had a function may apply to forms of decorations embedded in architectural components and to both changes of function of tectonic elements and chance of use of the architectural space. In the final part of the chapter, informal architecture will be introduced as a representation of architectural exaptation.