Construction sectors have a long way to go to realize a circular economy. Many organizational barriers and institutional characteristics inhibit the sector’s transition to circular practices. Nevertheless, within this early phase of the transition, several building projects were realized. This research aims to learn from these frontrunners, in order to distill insights on how to improve conditions at project level. Drawing on ecological systems metaphor, circular building projects in this research are regarded as a system of multiple actors that each perform one or multiple functions: these functions together make up the functional diversity of circular building project. The sum of these functions produces a system service (i.e. circular building elements). Using this perspective to analyze four circular cases, we uncovered five functions that are crucial to realize circular buildings: 1) connecting though vision; 2) matching supply and demand; 3) providing used materials; 4) constructing circular building elements and 5) controlling safety and quality. The functional diversity perspective reveals that functions are to large extent interchangeable between actors. Further applications of the functional diversity perspective could reveal its relevance to support the transition to a circular construction practice and possibly other transition themes. @en