Apartment buildings are a significant component of the built environment but their potential to contribute to city sustainability goals through building retrofits is often overlooked and unrealised. In Australia this can be due to complex governance arrangements arising from their multi-person ownership structures, exclusion from government assistance or from a lack of knowledge about what is possible both amongst owners and within industry. In order to better understand why retrofit remains a fringe practice for these buildings this research uses Actor Network Theory to trace several retrofit projects in Melbourne and Sydney. By identifying the individual actants in these successful retrofit processes the interconnectedness of the technical and social dimensions is revealed. This approach demonstrates how retrofit possibilities are shaped by the unique combination of a) the materials and design of the building, together with b) the capability and other resources of the owner group and c) the sustainability expertise in the local building sector, in the context of d) their particular neighbourhood. Having greater insight into these dynamics and the commonalities across projects could improve guidance for others seeking to improve the sustainability and liveability of their apartment buildings. Knowing what factors enabled the case study projects to succeed despite the numerous challenges any project of this type faces provides an important step in developing a pathway to sustainability retrofit of apartment buildings becoming mainstream practice. In turn, this would benefit not only the residents of these buildings but also their communities more broadly as improvement in the performance of existing housing stock is an essential element in meeting sustainability goals.