The envelope shape is considered the most salient design characteristic in a building and also impacts its energy performance. And although energy efficiency in design is gaining momentum, the value of architectural attributes should not be underestimated. Design rules of thumb are useful guidelines to achieve better energy performance but they do not substitute energy simulation results, particularly in more complex projects. However, in the early design stages, when the envelope shape is defined, energy information is normally inexistent, due to modeling for energy simulation being a time-consuming task, frequently overlooked at this phase, when the design is still uncertain regarding many parameters. This paper presents a methodology to assist design decisions regarding the building envelope shape taking into account its implications on the energy performance of the building. Basically, this methodology involves a design system, an editable (flexible) system to generate various envelope shape designs, with integrated energy simulation, to calculate the energy consumption values of each generated design. Design systems encoded into shape grammars are more adequate for the architecture field, given their ability to guarantee stylistic coherence, but they are hard to implement. This methodology converts a grammar into a parametric design system and is illustrated with its application to the grammar for Frank Lloyd Wright's prairie houses. Output examples of the application are also presented. Results show the validity of the proposed methodology and the addition of optimization techniques is identified as the obvious next step.