In the face of the climate emergency, Swiss environmental law has become an essential – and sometimes restrictive – framework for any construction project. This presentation explores the sources and fundamental principles of Swiss environmental and climate law, with a focus on the built environment. It covers the main areas of environmental law and is structured around concrete examples and practical cases designed to highlight the challenges and tensions that certain construction projects can raise: how can we reconcile densification, regeneration and environmental requirements? Where do the limits of the law lie in the face of the imperatives of the ecological transition?