Although urban plans may appear simple once completed, they are the outcomes of a highly complex decision-making process. This complexity makes it difficult to develop Planning Support Systems (PSS) that are comprehensive, flexible, intelligent and, yet, understandable to users. Great improvements are within reach, however, when observing the advances being made with regard to component-based software engineering and development and the role that multi-agent technology can play in this respect. This paper intends to bring this to mind and introduces a conceptual framework for PSS to describe how using a multi-agent paradigm as structuring principle could further both the intelligence and flexibility of such systems in terms of operation, functionality, usability, extensibility and adaptability. The framework is structured as an agent organization consisting of three types of agents: 'interface agents' to improve the user-system interaction; 'tool agents' to support the use and management of models; and 'domain agents' to provide access to specialized knowledge. As the agents are linked to specific system components, data flows in the system take the form of agent communication, while the interaction between agents enables a smooth integration of system functionality. Furthermore, the consistent definition of agents as human-like specialists facilitates users' understandability.