There is an increasing demand for higher quality office buildings. Occupants and developers of office buildings ask for a healthy and stimulating working environment. The advent of computers and other office equipment increased the internal heat gains in most offices. Highly glazed facades, often with poor shading, have become very common. This, together with the extra heat gains from the electric lighting made necessary by deep floor plans, and the wider use of false ceilings, increased the risk of overheating. Decisions taken rapidly in the early stages of design can have a large impact on the performance of the finished building. For example, choices of the overall form of the building, the depth and height of rooms, and the size of windows can together double the eventual energy consumption of the finished building. They can also halve the daylight levels, and increase summer temperatures to levels which affect the occupants' productivity. Later in the design process, radical design changes are rarely made. The paper uses data from practice and meteorological data for Belgium (northern part of Europe) to determine directions which should be used in practice. So, this research represents a help for architects to design energy efficient buildings with a good thermal interior climate.