This article is based on a research and development project conducted in one upper secondary school in Norway. The participating teachers have been part of a three year longitudinal study, and the findings in the present study are from their third and final year of a professional development study. The aim of this study was to find out how teachers perceive the impact of collaborative design on their professional development. My contribution to the field of research on Teacher Professional Development (TPD) is established on the assumption professional development take place when teachers have the opportunity to collaborate in designing their TPD. The research question is: "how can learning by collaborative design enhance teacher professional development?" The collaborative process of design provides openings for teachers to reflect on the intentions and implications of the projected development. Findings from the present study show how teachers in one department at a school design new collective practices in reflections on teaching. This design impacts other departments at the school in that these also implement the collaborative design in their professional development.