The sound development of city-regions presents a major planning challenge, as these regions are and will be the living spaces for the majority of the population. Therefore, a key question is how city- regions can be managed so that they develop in a sustainable way. Although Environmental Impact Assessment, Integrated Assessment, and other currently used approaches provide significant inputs for managing transition-processes towards sustainability, they must be extended to respond to three major deficiencies, which are (i) using lists of isolated indicators, (ii) not performing a consistency analysis of the targets to be achieved, and (iii) not utilizing the potential of transdisciplinary approaches. The authors present an approach to constructing Sustainability Solution Spaces for Decision-Making (SSP). This approach fulfils the systemic, normative, and procedural requirements of an appropriate sustainability assessment as elaborated in the technical literature. It provides a consistent set of targets considering the systemic relations among the indicators representing the city- region. This gives the decision-makers a concise guideline for sustainable decisions and makes them aware of the synergistic and contradictory effects of their decisions. The modular tool is first depicted as a general procedure and later differentiated into two transdisciplinary approaches, a participatory and an expert approach.