Product family design is a cost-effective way to achieve mass customization by allowing highly differentiated products to be developed from a common platform while targeting individual products to distinct market segments. Recent trends seek to apply and extend principles from product family design to new service development. In this paper, we extend concepts from platform-based product family design to create a novel methodology for module-based service family design. The new methodology helps identify a service platform along with variant and unique modules in a service family by integrating service-based process analysis, ontologies, and data mining. A function-process matrix and a service process model are investigated to define the relationships between the service functions and the service processes offered as part of a service. An ontology is used to represent the relationships between functional hierarchies in a service. Fuzzy clustering is employed to partition service processes into subsets for identifying modules in a given service family. The clustering result identifies the platform and its modules using a platform level membership function. We apply the proposed methodology to determine a new platform using a case study involving a family of banking services.