Historical buildings are being destroyed over time and energy losses are increasing. Therefore, energy efficient preservation of historical buildings is an important issue. However, the application of contemporary additions has increased in cases such as the revival of building units that have not survived to the present day or when a new post-functional space is required. The aim of this study is to evaluate the impact of contemporary additions on the energy performance of historic buildings through a case study. For this purpose, energy simulation analyzes of the historical Süleyman Pasha Bath in Kocaeli province were performed through Design Builder. Before the simulations applied, information about stone, which is the original material of the building, and glass applied with contemporary materials were entered into the programme. The provinces of Izmir, Konya, Sivas, and Erzurum were selected from five climatic regions for the contemporary additional analysis. In these provinces there are many traditional bathing buildings with similar plan types. According to simulated results, it was concluded that the application of modern additions after the restoration negatively affected the energy performance in all five climate zones. Before applying contemporary additions to historical buildings, factors such as the microclimate, material properties and geometry of the building should be taken into consideration during the design phase and a decision should be made as a result of various analyses. Consequently, when contemporary additions to historic buildings are required, using the most effective construction techniques and materials is important in terms of building sustainability and effectiveness.