The reprocessing of used products within a circular factory relies on instance-individual design decisions. This requires specific design knowledge (SDK) on relations between embodiment and functional behavior. However, existing approaches do not model SDK in a way that supports product reuse to fulfill the functional requirements of new product generations. This paper presents a hypothesis-based modeling approach on building and structuring qualitative SDK. Drawing on elements of existing product models, the approach yields three outcomes - a function-related structure, design hypotheses, and the assignment of testing strategies. A case study of an angle grinder demonstrates how the approach addresses the requirements of a circular factory by facilitating targeted SDK buildup, ensuring comprehensive documentation, and preparing the quantification of knowledge.
