Today, cities worldwide are engaged in urban projects and activities in a concerted drive towards sustainable development. However, the concept of “sustainable urban projects” is inherently normative, subjective and ambiguous. Furthermore, the popularity of sustainable urban initiatives does not guarantee that increased pressure on dominant unsustainable urban systems will occur. In this article, we argue that strong urban debates on these initiatives and on urban sustainability are required to facilitate and stimulate urban systems towards a more socially just and environmentally sustainable future. When we say “urban debates” we mean substantive talks and detailed discussions about the type of cities we want to live in and about a shared understanding of sustainable urban projects and how they affect urban systems. We aim to contribute to that objective by developing a discussion framework on sustainable urban projects that frames sustainable development as a challenge that concentrates on both ecological and social concerns and that avoids a sole reliance on technology fixes. But above all, we also incorporate insights and findings from transition thinking to focus on radical changes or transformations of urban systems and to acknowledge the importance of so-called “niches”. In this article we describe the fundamentals, the surplus value and the utility of the framework. The article contains empirical material from a pilot-study in Ghent, Belgium.