Higher requirements in quality management, in the reduction of design time and manpower as well as more extensive, life-cycle spanning documentation are resulting in an increasing demand on modern information technologies. Computer-aided, integrated, knowledge-based design systems are an approach to build a new generation of IT-support for plant design and optimisation. As a contribution to this field a design system for computer-aided piping layout is being developed. Piping design is subject to many rules and constraints whose violation or satisfaction have to be described and weighted. Therefore routines generating multiple feasible alternatives for pipe routes are programmed. In a rating and decision system (numeric-heuristic system) these alternatives will be evaluated and ranked.