In the project “City as Space of Possibilities“ , we have dealt with sustainable urban development possibilities from different disciplinary perspectives, researching artistic-creative and socio-economic projects and initiatives, as well as larger institutions such as the city administration (a case study in Hanover, Germany). A large number of urban actors are turning their attention to sustainable urban development, many of them from the cultural sector. The spectrum of activities ranges from the renunciation of a growth-oriented economy to the creation and consolidation of networks, the strengthening of citizen participation, the mediation of values and ethical actions, and the realization of ecological, social, cultural, and alternative-economic objectives. Many actors are part of the arts and the civil society. Among these actors, we were able to identify "entrepreneurs of conventions": They change prevailing rules and conventions in such a way that the actors of sustainable urban development are not only active in niches, but can also make the structures and processes of the city, as a whole, more sustainable. Many actors, however, remain in small niches and try, rarely successfully, to influence the structural "texture of the city" in a "bottom up" manner. Instead of niches, we speak of "spaces of possibility", i.e. protected experimental spaces in which the rules of serendipity can be used creatively, and often playfully, to develop and apply new institutions of sustainable urban development. Solidary based on trust in networked spaces can lead to new sets of rules for comprehensive, sustainable behavior and attitudes.