In recent years, cities have revealed themselves as being prominent actors in the circular economy transition. Besides supporting and initiating urban projects catalyzing circularity, cities are looking for monitoring tools that can make their progress towards circularity visible. Adopting Leuven’s pilot project for a building materials bank as a case study, this paper notes the particular challenges and opportunities in the pilot project to assess its progress and impact, in combination with gathering data for overall circular city monitoring purposes. Firstly, the paper names tensions between the “messy” transition process from policy ambitions to implementation and the question of data and monitoring. Secondly, the paper identifies relevant dimensions and scales to evaluate progress and impacts of a building materials bank, drawing from its development process. Thirdly, it proposes guidelines to monitor and evaluate circular city projects from the bottom up, combining quantitative indicators with guiding questions in a developmental evaluation. The analysis serves a critical reflection, distills lessons learned for projects contributing to circular cities and feeds a few concluding policy recommendations. The case study serves as an example that, in order to move beyond the tensions between circularity monitoring and actual circular city project development, monitoring instruments should simultaneously interact with and feed the circularity transition process. Therefore, dedicated data governance driven by enhanced stakeholder interactions should be inscribed in transition process guidance. Bottom-up projects such as a building materials bank provide opportunities to do this.