Urban Design processes need to adopt flexible and adaptive procedures to respond to the evolving demands of the contemporary city. To support such dynamic processes, a specific design methodology and a supporting tool are needed. This design methodology considers the development of a design system rather than a single design solution. It is based on patterns and shape grammars. The idea is to link the descriptions of each pattern to specific shape rules inducing the generation of formal solutions that satisfy the pattern. The methodology explores, from the urban designer point of view, the capacity of a shape grammar to codify and generate urban form (Duarte et al, 2007). This paper defines the ontology of urban entities to build on a GIS platform the topology describing the various components of the city structure. By choosing different sets of patterns the designer defines his vision for a specific context. The patterns are explicated into shape rules that encode the designeris interpretation of the pattern, and operate on this ontology of urban entities generating solutions that satisfy the patternis concept. Some examples of the topological relations are shown.