This paper presents the situated Function-Behavior-Structure (sFBS) model of co-design, developed within the FBS ontology. In co-design, designers interact with their co-designers and with their own cognitive experiences. In this model, we describe a representation of the overall co-design activity, while preserving a fine-grained representation of each designer’s interactions with their co-designers and with their internal cognitive processes. The relevance and potential of our model are illustrated through multiple examples.