In this research, an exemplary study has been conducted in order to draw attention to the importance of evaluating the existing potential on the roofs of buildings. This research offers an evaluation of the existing solar energy and rainwater potential on the total roof area of the buildings in the Izmit district, which is a central district of Kocaeli province, one of the busiest centers of industry in Turkey. The calculations in this study were carried out by using the data obtained from various institutions. As a result of these calculations, the ratio of electrical energy that can be provided with photovoltaic systems on roofs to meet the annual electricity consumption of the district was found to be 203.581%, and the annual solar energy utilization rate for a family of 4 to bring 240 liters of daily use water temperature to 60 degrees C with an 8 m(2) collector area was calculated as 66%. In addition, the ratio of rainwater that can be collected from the total roof area of the existing buildings in the district to meet the domestic water consumed by the district was found to be 33.27%.