The GSW Headquarters, one of Berlin’s most recognisable buildings, was refurbished and extended with an outstanding architectural and urban solution, using innovative mechanical engineering concepts and based on natural ventilation. These concepts were integrated into a specially developed structural solution. The overall aim for the mechanical design was to improve the building's sustainability rating by achieving energy savings of 30-40% in comparison to an ordinary building. The characteristic low-energy design comprises a double-layered façade and cross ventilation. The western façade has a second glass skin which ventilates and cools the building, dispelling hot and used air. In addition, the double facade serves as a second buffer for thermal and acoustic variations. One result of these design interventions is that the building benefits from energy savings of up to 40% in comparison to traditional designs. The integration of building services into the floor slabs allows long-span spaces and maximum clear height in the office zones. Suspended ceilings were not necessary; fair face concrete with embedded lighting, sprinkler and smoke detection systems provided a futuristic image.