The transition toward sustainable cities has garnered significant attention from governments, policymakers, and researchers. Mirroring these developments in the academic world, organizational researchers are investigating sustainable placemaking and identifying the conditions that favor such a vital process. Building on a case study of an urban planning committee in the Swiss town of St. Gallen, this research studies how spaces (temporary social settings that enable the negotiation of new ideas) can initiate sustainable placemaking. The interplay between the expansion of space members’ role identities and their imagination of sustainable places is essential to sustainable placemaking. Generalizing these findings, our process model sheds light on the mutual constitution between imagination and role identity expansion inside spaces. The model makes two contributions to the organizational literature: (i) it highlights the importance of role identity expansion in sustainable placemaking processes through spaces and (ii) it discloses the interplay of imagination and identity expansion in spaces.