Cognitive Work Analysis (CWA) is an appropriate approach in design for high-stakes domains, such as air traffic management (ATM) since it focuses on human expert performance in regular and contingency situations. However, CWA is not suitable for the design of a first-of-a-kind system since there is nothing to analyse before the start of the design process. In 2017, unmanned air traffic management (UTM) for intense drone traffic in cities was such a system. Making things worse, the UTM system has to be in place before the traffic, since it provides basic safety. In this research-through-design study, we present conceptual designing as a bootstrapping approach to CWA in the design of a first-of-a-kind UTM system. In a series of co-design workshops, we identified future services, traffic patterns, and regulations that framed the design of UTM system concepts. They were based on combinations of four basic building blocks: points, lines, planes, and volumes. Concepts of point-based control, airport geofences, grid squares, layers, and tubes were discussed. Throughout the conceptual designing, results were documented in an evolving Work Domain Analysis (WDA), which is a cornerstone of CWA. This approach allowed us to bootstrap the CWA for a first-of-a-kind-system.