Architects arguably have the greatest influence on the design of buildings. One of the key factors that make it hard to improve the energy efficiency of buildings is the use of architects’ reasoning by architects at the early design stage. There is a need to assess the impact of architects’ reasoning on the energy performance of the designed building. To this end, this research was conducted in two phases. Firstly, the most influential design parameters, in terms of energy efficiency, were identified and used to develop a design exercise issued to a sample of practising architects in the north of Algeria. Design exercise participants were required to minimise expected energy consumption along with the construction cost. Secondly, computer-generated dynamic design optimisation for the same design task was conducted in DesignBuilder v6. 1 .8. The computer-generated designs decisively outperformed the human-generated designs. The experienced architects achieved the least-performing designs rather than those with less experience.