The concept of the circular economy (CE) encompasses various strategies. One of these is the adaptive reuse of historic buildings, which is mostly used in the restoration of historic buildings that have lost their original function. This study first aims to explore in detail the core principles of circular economy and its relationships in the field of cultural heritage conservation in order to provide the sustainability of cultural heritage. It then examines and illustrates the application of these circular economy principles as a multi-perspective tool to preserve the value of a modern monument in Ankara, Türkiye. The PTT Stamp Museum in Ankara, built in 1933–1934, was chosen as a case building to analyze and demonstrate the relationships between modern architectural heritage, retrofitting, heritage conservation, and circular economy principles during the final restoration works. Originally built as a bank and designed by one of the most famous foreign architects of the time, Clemens Holzmeister, the Emlak and Eytam Bank was last restored in 2013 and now serves multiple cultural purposes; museum, café, play area for children, seminar room and exhibition space. The study design includes a literature review, archival research, and field studies. The results show that the case study building has incorporated circular economy principles to some extent in the recent restoration work and that it has created new jobs, transformed the region in which it is located, and created new value for cultural development thanks to its adaptive reuse. Therefore, this study is important to demonstrate the social, environmental and economic benefits of the circular economy in the conservation of modern cultural heritage.