A flexible product architecture enables easy future changes to the product. We introduce here a procedure to design products using existing modularity methods and a novel redesign effort complexity metric that helps define module boundaries so that changes in the modules require minimum redesign effort. The metric is based on redesign difficulty of material, energy, and information flows. We find that different interface types need different amounts of redesign effort in order to accommodate a change. We show the use of our method through an example of a gas sensor. © 2004 Elsevier Ltd. All rights reserved.