Design engineers working in design and planning of urban and industrial areas are confronted more and more frequently with complex solution spaces, filled with hundreds of alternative combinations of possible sub-solutions supplied by a host of specialists. As a result of this, their quest for the optimum design tends increasingly to run aground in too many options, too many opinions and too many alternatives. This paper sets out an approach for decision-based design by means of the combination of sub-solutions. Ideas from management theory and operations research, and mathematical models which make these ideas operational, can aid in bringing the design and planning process to a successful conclusion. The focus is on a collaborative approach to design and urban planning, and project management. While the design objects are modeled in computer models, the decisionmaking process is modeled in the interactions of the stakeholders with the tools and each other.