Architects are the key actors in the building design process and as such responsible for many important design decisions, of which a large share influences the final sustainability level of the building. Up to now, little attention has been paid to their handling of (sustainable) material use, since no related regulation exists yet (in contrast to energy performance regulations). Some countries like the Netherlands are preparing for future legal requirements on the environmental impact of buildings, by imposing a mandatory calculation of the environmental impact of new buildings. Also the Flemish/Belgian authorities are developing a tool to incorporate environmental impact calculations in the building design process. In anticipation of these upcoming legal requirements for sustainable material use, the design process of 14 real life design projects has been unravelled with a specific focus on the material choices during the design process. A process scheme of the material choices along the different design phases has been developed. Semi structured interviews with the architects gave input to fine tune the process scheme. This paper presents the lessons learned from this analysis in view of tool development for and feedback on the environmental performance of building design directed to the architect. ; Research funded by a PhD grant from VLAIO (Agentschap Innoveren en Ondernemen, previously called IWT-Vlaanderen).