The renovation of the WiSo High-Rise at the University of Cologne demonstrates how circular construction, climate protection, and modern functionality can be successfully combined. Guided by the principle “The Best Building Material Is the Existing Structure,” the project preserved and transformed a listed university building while maintaining its architectural identity. A key objective was to maximize the reuse of existing materials and components, including concrete, brick, steel, natural stone, wood, and interior fittings. By treating the building as a valuable resource, the project significantly reduced embodied carbon emissions and consistently followed the principles of sufficiency and circularity. At the same time, the high-rise was upgraded with state-of-the-art building systems, energy-efficient technologies, improved insulation and glazing, as well as modern fire protection standards. These interventions enhanced performance without compromising the building’s original character. The design is based on three core strategies—Building DNA, Adaptability, and Grey-to-Green—which support long-term flexibility and sustainable use. By making the transformation visible, the project creates a tangible connection between past and future, resulting in an educational building that combines heritage, functionality, and sustainability.