There has been a shift in aesthetics from the modern orthogonal building envelope to more elaborate curved and folded forms. Non_orthogonal forms are often associated with complete freedom of geometry, entrusting the advancement in custom manufacturing and robotic fabrication of one-off building parts to realise the design. This paper presents a methodology that allows non_orthogonal surfaces to be designed using a constrained library of discrete, tessellating parts. The method enables the designer both to produce ‘approximations’ of freeform designs in a top_down manner or to generate ‘candidate’ designs in a bottom_up process. It addresses the challenge in the field of design engineering to generate architectural surfaces which are complex, yet simple and economical to construct. The system relates to the notion that complexity derives from simple parts and simple rules of interaction. Here complexity relates to the holistic understanding of a structure as an interaction between its local parts, global form and visual, as well as functional performance.