The paper proposes an extension of the Function-Behaviour-Structure (FBS) framework to multi-levelc design representation. The ontology based on function, behaviour and structure has been enriched with a new design entity, the topology, with the aim of connecting more levels of representation. According to this new paradigm, design activity is not focused exclusively on working principle, shape and material at macro level, but goes beyond, to greater levels of detail, designing for example how to dispose material in the inner structure of the product parts at microscopic level. Structural optimizers are excellent tools to design the topology of a structure according to its function and behaviour, but they have been conceived for working only at mono-level. This paper proposes a multi-step optimization process for improving the versatility of structural optimization tools allowing them working also in both macro and microscopic dimensional scales.