The old office building at Groot Willemsplein, Rotterdam, dating back to the 1940’s, was renovated to give it a new life with commercial functions on the ground floor and flexible office spaces at the other floors. The most important climate adaptation and mitigation measure implemented is the energy-efficient cooling and heating system. An Aquifer Thermal Energy Storage (ATES) system supplies the building with heat and cold. In summer, heat will be absorbed and stored in a ground water aquifer, this stored heat can be used in winter to heat the building.