When planning to carry out an activity, a mobile robot has to choose its placement during the activity. Within an environment shared by humans and robots, a social robot should take restrictions deriving from spatial needs of other agents into account. We propose a solution to the problem of obtaining a target placement to perform an activity taking the action possibilities of oneself and others into account. The approach is based on affordance spaces agents can use to perform activities and on socio-spatial reasons that count for or against using such a space.