This paper presents a new Generative Design System to be used by architects in early to intermediate stages of design, to help improving the environmental performance of buildings. Both thermal and lighting analysis are included in the system, together with methods to incorporate architectural design intentions into the generative process. The Generative System was applied to a building by Alvaro Siza in Oporto (Portugal), to test its capability to handle complex architectural designs and to generate solutions within given language restrictions, while still reducing energy consumption levels of the building. Variables studied were fenestration design, shading systems and building shape (roof geometry). The advantages of using Rapid Prototyping technologies coupled with this Generative System are discussed, and an example of the application of a FDM (Fuse Deposition Modeling) 3D Modeler to this specific study is presented.