Facing the difficulties of interoperability and cooperation between several urban databases, a solution is based on ontologies which can help not only clarify the vocabulary used in urban planning, but also organize urban applications; indeed multiple definitions can be given to various urban objects. So this is the scope of the Towntology project which aims at defining ontologies for urban planning whose design is characterized by the multiplicity of definitions. After having presented some ways of using ontologies for various actors in urban applications, a definition of pre-consensus ontologies is given, together with some groupware tools to collect multiple textual and multimedia definitions in sub-ontologies, to check and consolidate the vocabulary in order to reach some consensus. We conclude this paper by giving some recommendations for the Towntology project for covering the whole urban field by integrating various sub-ontologies.