Citizen participation is often a goal of urban development. However, in reality levels of actual participation are often low, limited to certain sets of stakeholders, and the collaboration between them often poor. In engaging with this dilemma, we have established a new research project, Co-constructing city futures (3C), which addresses current challenges to citizen participation in contemporary urban development. The project sets out to encourage and facilitate a shift from processes that intentionally or unintentionally exclude certain groups. Building on ideas about collective learning, it inquires and experiments with public debates and planning processes to enable citizens and other stakeholders to engage in the co-construction of ideas and visions for city futures. Democratic design experiments focusing on green mobility and blue-green infrastructures will be carried out in selected Norwegian cities in collaboration with planners, citizens, and other actors to identify their needs, challenges and interests, and to develop, test and evaluate ideas and prototypes for digital tools that can be applied both in Norway and in other global cities. As a point of departure, we pose three critical questions regarding citizen participation in urban development, with hopes that this can invigorate discussion around this emerging research agenda.