Retrofitting the widely used prefabricated concrete buildings can reduce energy consumption and improve occupant comfort in winter and during the increasingly frequent summer heat waves caused by climate change. The goal of this research was to find the optimal retrofit measures for a typical prefabricated concrete building in Hungary based on energy consumption and summer comfort. The building was studied with dynamic simulation, calibrated with real consumption data. Different retrofit alternatives such as thermal insulation, window replacement and shading were investigated. A sensitivity analysis was used to identify the most important factors, followed by optimisation to investigate how the different retrofit options interact with each other, which retrofit methods have the greatest impact on reducing the number of discomfort hours and which factors have the most significant impact on energy consumption. For the optimisation, the genetic algorithm of DesignBuilder was applied. As a result of the study, the optimal retrofit solution was selected from a number of alternatives and its effectiveness was analysed using future climate models. Based on the results, recommendations can be made for retrofits that reduce greenhouse gas emissions and improve occupant comfort. In the long term, building modernisation contributes to sustainable development and the fight against climate change.