Used by diverse actors to justify the implementation of new participatory processes, the notion of “citizen knowledge” remains theoretically abstract. Works of political sociology have approached this question in a global and theoretical way: by joining in on a classic political science debate on the political competence of ordinary citizens and by drawing inspiration from recent reflections of science studies on the notion of “technical democracy.” To define more exactly the epistemological contribution of ordinary citizens in local public action, this paper focuses on urban policies and rests on an empirical approach. It leans on an ethnographic study — led during three years in Paris — in order to define the nature and status of citizen knowledge, in connection with the questions of legitimacy and power, in the processes of participatory town planning. We observe that the circle of knowledge is opened and that borders between expert and profane knowledge in the urban field are reconfigured. Far from being limited to the local knowledge for which they are sought, individuals and collectives can also mobilize a technical expertise and militant knowledge. However, this democratization of access to knowledge and to power remains limited because social inequalities persist in access to these participatory processes and in the capacity of participants to draw from a diversified range of knowledge.