Making sound decisions early in the design process - for example on the shape of the building and its rooms, and on the size and position of windows, and the choice of glazing - can reduce energy consumption by up to 50 percent. They can also double daylight levels and avoid overheating in summer. The design tables in this guide predict the maximum internal air temperature on a hot summer day, and the level of daylight on an overcast day in winter, for common cases such as rectangular rooms with a window in one or both ends. The guide also describes - in concise plain English with 150 illustrations - the principles for designing well daylit offices which will not overheat in summer.