Socially aware robots have to coordinate their actions considering the spatial requirements of the humans with whom they interact. We propose a general framework based on the notion of affordances that generalizes geometrical accounts to the problem of human-aware placement of robot activities. The framework provides a conceptual instrument to take into account the heterogeneous abilities and affordances of humans, robots, and environmental entities. We discuss how knowledge about (socio-)spacial aspects of affordances can be used in various reasoning tasks relevant to human-robot interaction. Applying the notion of a practical reason, socially-aware robots are able to solve the social activity-placement problem.