This paper presents a shape grammar for planned urban spaces intending an implementation for generative urban design. This implementation will form part of an urban design support tool defined to formulate, generate and evaluate urban designs. The goal is to formulate urban program descriptions according to context conditions using a description grammar and generate alternative design solutions using a shape grammar. The generation is guided by several evaluation processes performed by an evaluation module. In this paper we are focusing on the definition of the generic shape grammar using an existent urban plan as a case study. The aim is to encode the design moves of the urban designer into generic grammar rules amenable for specific instantiations through the customization of rule parameters.