Citizens’ initiatives are introducing and testing novel planning approaches and are therefore conceptualized as a potential source of social innovation. We explore the social innovation processes through a focus on institutional change. The research project analyses how citizens’ initiatives develop and test novel ideas and approaches (emergence of innovation), how these ideas and approaches are evaluated, shared, and picked up by other public and private actors (diffusion of innovation), and how they in the end impact the institutional frameworks of urban planning and therewith contribute to a transformative form of change (impact of innovations).