AbstractDesigners, in response to codes or voluntary ⬓green building⬽ programs, are increasingly concerned with building energy demand reduction, but they are not fully aware of the energy saving potential of architectural design. According to literature, building form, construction and material choices may be powerful drivers of energy efficiency ⬜ but a very few studies have quantified their actual effect in different climate, and none of the study is based on today computational possibilities. This research was inspired by, and attempts to verify, the ideas from two of the most influential books on sustainable design: ⬓Design With Climate⬽ by Olgyay (1963), which discussed strategies for climate-adapted architecture, and Lechner̽s ⬓Heating, Cooling and Lighting⬽ (1991), on how to reduce building energy needs by as much as 60 ⬜ 80 percent with proper architectural design decisions. Both books used results from building energy simulations made with limited computational resources available at the time. The research presented in this paper uses a genetic algorithms based approach for the optimization of heating, cooling and lighting energy demands of different building designs. In total, over 25 million different buildings constitute the optimization search space, and the most energy efficient design solutions were explored for 8 different climate zones. The building designs are varied by shape, orientation, window to wall ratio, component and construction types, materials, and different occupant behaviour. The research shows the best solution for each of the climates and compares them with Olgyay̽s findings. Finally, for each climate the energy saving potential is defined and then compared to Lechner's conclusions.