The ultimate goal of the project in which the described research evolves is a computer-platform for assisting in the development of urban design proposals at the site planning scale including formulation, generation, and evaluation modules. The paper is concerned with the development of a common ontology that guarantees inter-operability among the three modules. After clarifying the theoretical framework, it describes the methodology used to develop the proposed Urban Design Ontology (UDO), as well as its lexicon, syntax, and semantics. It is argued that such ontology constitutes the bases for a City Information Model (CIM) that permits to elaborate consistent and comprehensive descriptions of urban spaces, thereby enabling the formulation of programs for urban interventions and the generation of corresponding design proposals.