This study is concerned with the formulation of solutions for urban problems. It departs from Alexanderis pattern language theory and urban design guidelines, to create a system for generating specifications or the ingredients of a plan, given a scale, a site and a community. It takes into account strategies, regulations, guidelines, physical features of the site, and furthermore, the social, cultural and economic characteristics of the population. This system, sorted by a sequence of events, through stages, categories, methods and agents, describes taxonomic levels and their inner relations. Such an ontology provides a pattern encoding structure towards a computational model within the capabilities provided by the spatial data modelling of GIS (GIS-O). The urban formulation model is conceived to increase qualitative inputs, reducing ambiguities, through a flexible while automate process applied to urban planning.