Many recent modular design methods and approaches have focused on the modularization process - decomposition and composition. This paper suggests an integrated methodology that includes additional tools and stages for a complete modular architecture design. The borders of the modular design process are expanded by adding strategic issues, appropriateness to modularity, degree of modularity and modularity strategies, in an operationalized manner. The methodology presents a "requirements analysis-decomposition-composition-design evaluation" structure. The "modularization process" is designed so as to choose from three different perspectives - customer-based, function-based and structure-based design. To test and validate the methodology it was applied to a domestic gas detector product family. As a result, a new modular product architecture with eight modules was developed.