The durability of buildings plays a key role for the sustainability of cities and the communities inhabiting them. However, the progressive loss in the last three centuries of the ability to alter buildings - in favor of a more controlled design and operation - brought about a ‘static’ rather than ‘durable’ building stock. This manifests itself today on the one hand in buildings that poorly accommodate the speed of the societal change in lifestyles, and on the other hand in a new highly customizable ‘placeless’ architecture, resulting from decades of experimentation around the concept of ‘flexibility’. Hence, to overcome the gap between physical and social durability, research on the systemic interactions between the material and social components of the built environment is needed. Previous works shows that these interactions in the housing sector are regulated by the housing function, the most crucial determinant of the housing’s system dynamics. In this paper, we introduce the housing function as a new key to understanding the concept of flexibility in architecture. We do so by exploring, in an online survey (N = 878), the role played by housing functions in the decision of Swiss tenants to move out or in a dwelling. Our analysis indicates that the match between the desired and actual housing function corresponds to a higher level of satisfaction of the tenants with their dwelling. Secondly, it appears that specific life-events (e.g., divorce) correspond to a change in the desired housing function. Lastly, these housing functions can be linked to common features characterizing the dwelling (e.g., balcony). Thus, flexibility can be defined as the ability of a dwelling to fulfill more than one desired housing function. Following the need to comply with new sustainable building standards, we propose to assess and rethink the long-term sustainability of the current and future housing stock based on the proposed functions and features.