Design is an ill-structured and costly problem whose complexity can significantly increase during the process due to emerging requirements. How can the computer help the designer during the early-stage design of structures? How should they communicate together? How can the machine provide instant feedback on performance requirements, allow for fast generative exploration of alternative solutions and be capable of unveiling unexpected structural typologies? Our approach to solve this challenge is to let the designer operate on not-yet-complete structural systems while being informed by the computer of what would be the best next moves. Grammar rules are used as the common interface between user and machine in order to control both interim topologies and geometries. The approach fosters a close collaboration between user and machine, ensuring that the design process benefits from the experience of both the former and the latter.