Software is in constant evolution and many approaches have been suggested to study software maintenance productivity. This research reports on a process to design and implement a productivity model of legacy software based on the measurement of small functional enhancements using the COSMIC ISO 19761 international standard. Two motivations influence this research: 1) understanding the productivity of the software maintenance process to help manage the cost of maintenance; 2) understanding the cost drivers that affect the software maintenance productivity. This research reports on an empirical study of a productivity measurement program implemented in a large banking legacy system.