Tertiary sector buildings and office buildings in particular are heavy users of energy and hence have the potential to make significant improvements in their energy efficiency. To achieve this there needs to be a rethinking of the building design process leading to an optimization of the building’s energy demand and good indoor environmental quality conditions. The right decisions have to be taken in the early stages of design in order to achieve the best possible energy performance of the building. The main objective of this paper is to present the results of research on the parameters that most influence the building envelope’s energy performance for Mediterranean and Oceanic climatic conditions, according to the Köppen climate classification. The study investigates how two factors-thermal mass and window-to-wall ratio-influence a building’s energy performance. A parametric study on those variables is carried out through a dynamic simulation in order to evaluate their influence for Thessaloniki, Greece, and Nicosia, Cyprus-which both feature a Mediterranean climate-and London, United Kingdom, and Munich, Germany-which both feature an Oceanic climate. The results are discussed and conclusions drawn on the influence of each parameter.