This article presents and discusses the results of the Stakeholder Workshop (Co) Designing for Quality of Life: Exploring Challenges and Opportunities, which was held at Middle East Technical University (METU) in Ankara in October 2022 in the framework of the COST Action CA18137 European Middle Class Mass Housing (MCMH-EU). The workshop aimed to discover the possibilities of participatory design as a tool to address the necessary updating of the housing complexes of the Modern Movement (MoMo). The workshop, which was conducted on a cooperative housing estate, namely Ümitköy Sitesi, Ankara, Türkiye (1970), was carried out in five groups with members of different nationalities, ages, and experiences. This article argues that the public and private strategies which were followed to rehabilitate these complexes by focusing on the technical problems (construction pathologies, energy inefficiency, accessibility, parking, among others) tend to neglect, even ignore, the diverse social aspects involved. As a group of participants of this workshop, the authors of this article consider the involvement of all parties (experts, residents, housing management cooperative, and municipality) in the improvement processes of such middle-class mass housing sites as the key instrument to make these neighborhoods more inclusive and sustainable. This article evaluates the Stakeholder Workshop’s co-design performance as an instrument to improve the Quality of life (QoL) and sustainability of the neighborhood. The critical analysis of the workshop results leads to several significant conclusions: Social aspirations do not always coincide with political and technical ones; technical rehabilitations are not sufficient for the total improvement of QoL and sustainability of communities; (Co-)Design may have to be approached from different perspectives and, consequently, have different results; citizens have a great potential to participate and contribute to the improvement of QoL with innovative ideas and actions of different scales. However, the socioeconomic diversity of the inhabitants and restrictive legislation are the difficulties to be considered.