Traditional urban plans have definitive design systems, without the flexibility required to deal with the complexity and change that characterise  contemporary urban societies. To provide urban plans with increased flexibility, it is proposed a design methodology capable of producing various design solutions instead of a specific definitive design. The methodology uses shape grammars as a process for generating urban design. In this approach, design becomes a system of solutions rather than a specific one.  Through the analyses of a group of urban plans, a design methodology was sketched in which rules are used to enable more flexibility. These plans where chosen for their perceived qualities in terms of language, planning efficiency, and latent flexibility. As a result, a four-phased methodology was identified and thus, proposed for designing urban plans. This methodology was then combined with shape grammars and tested in a design studio setting. Students were asked to use the methodology and shape grammars as auxiliary instruments in the design of a flexible plan for a new town. In the following year, to simulate real-world conditions and oblige students to consider urban ordering and scale, work was structured differently. First, students were asked to develop a rule-based urban plan as in the previous year. Second, they were asked to conceive a detail plan for a sector of an urban plan defined by another group of students following its rules. The plans were then analysed with the goal of refining the methodology.  Results show that shape grammars produce urban plans with non-definitive formal solutions, while keeping a consistent spatial language. They also provide plans with explicit and implicit flexibility, thereby giving future designers a wider degree of freedom. Finally, they provide students with a concrete methodology for approaching urban design and foster the development of additional designing skills.