This article proposes a model dealing with the synchronisation of system design process and project planning process. This synchronisation is named synchronous coupling and is based on the assumption which states that a mapping exists between system Bill Of Materials and projects Work Breakdown Structure. Synchronous coupling ensures a consistency in the monitoring and the control of the system design and project planning by imposing synchronous milestones during the project development. The synchronous coupling is based on the definition of three states for each design and project entity following feasibility and verification criteria, and can thus manage the changes between states. Some rules are defined (precedence and coupling rules) to forbid certain changes when they are inconsistent and to synchronize them.