Prototyping is important for design exploration. While various computer-aided conceptual design systems (CACD) aim to support this practice, they are somewhat limited in their ability to suggest interpretations in-context. To improve these systems, we need a better understanding of how designers interpret things when designing, and what factors influence this activity. We observe architectural designers’ design process, and conducted a deep analysis of the activity at several levels, to capture interpretative events. The analysis of these reveals interesting patterns of design interpretation, which may be used to enhance future CACD systems.