This paper explores how opportunities for reducing the total use of office space can be identified, investigates how the benefits in terms of energy savings from space efficiency measures could be calculated, and gives a first estimate of such values. A simple method to measure office space use is presented and tested at two university departments, and very low space efficiency is found. A variety of reasons for the low space efficiency are identified via interviews with property managers and heads of the concerned departments. These include the fact that the incentives for using space efficiently are small for the decision-makers, and the costs in terms of time and trouble are perceived as high. This suggests that interesting results can be achieved without large efforts. Moreover, we present a proof of concept of how to estimate the amount of energy that can be saved by reducing space use. We find a rough estimate of the potential energy savings of 2 MWh/m2 in embodied primary energy intensity (assuming that more efficient use of space leads to a decrease in new construction) and 200 kWh/m2/year in final energy intensity. Those numbers should be useful as rough estimates when looking at opportunities for saving energy by using space more efficiently.