The lifespan of buildings has been extended due to technological developments. It is predicted that 75%-90% of the existing buildings will continue to be used in 2050. Buildings have an important place in total energy consumption and carbon emissions. With the measures taken in buildings, it is possible to reduce energy consumption by 25%-40%. In this study, we analysed ways to reduce the energy consumption of existing buildings by taking into account the effects of climate change. In the study, a numerical study was conducted on reducing PEC and CO2 emissions in existing buildings. The suggestions for the buildings were created based on the optimum building envelope, mechanical system, and building form group. These optimum suggestions were optimised with the NSGA II algorithm, taking into account the climate change scenarios from 2020 to 2080. As a result, the province with the lowest decrease in PEC and CO2 emissions was Kirikkale (PEC 36%, CO2 33%) and the province with the highest number was Isparta (PEC 69%, CO2 75%). Regionally, the region with the lowest decrease in PEC and CO2 emissions was the Aegean Region (PEC 41%, CO2 42%) and the region with the highest number was the Mediterranean Region (PEC 68%, CO2 72%).