Office buildings characterized by open-space offices and large windows generally have thermal comfort problems. In the summer period, the amount of thermal discomfort depends mainly on overheating due to thermal loads, such as electrical devices, occupation, and solar radiation entering through the windows. In Mediterranean climate, summer thermal gains are extremely high in these types of buildings, and to achieve the conditions of indoor thermal comfort, air conditioning systems are used excessively and waste a large amount of electrical energy. Efficient solutions to define optimal comfort conditions include passive design strategies such as window shading, and natural ventilation operated by a Building Energy Management Systems (BEMS). The objectives of this paper are to evaluate the conditions of users’ comfort in an office building located in Madrid (Spain) and to design a new operational strategy by using the existing solar shadings to control the solar radiation and by using hopper windows to increase natural ventilation. The results of numerical simulations in DesignBuilder highlight an average internal temperature reduction of one degree and an improvement of comfort conditions in the occupied period.