Design engineers and process planners need to search for similar designs. Design engineers use similar designs to estimate a new design's manufacturability. Like process planners, who need to generate process plans before production begins, design engineers can use an existing, similar design's plan to create a new process plan. Then, they can evaluate the new design. Variant process planning, a common process planning approach, uses a design similarity measure to identify the most similar design and retrieve a useful process plan. However, standard design similarity measures do not explicitly consider the production process. This paper presents an approach for developing a new class of plan-based design similarity measures. Such a measure explicitly exploits process plan similarity and thus improves the variant process planning approach. An example illustrates the approach and compares the new measure and a traditional group technology code-based approach.