This article analyses sustainable dimensions of intergenerational homesharing by exploring an ecology of sustainable relations of this housing service and various spatial scales in France. Despite the solidarity objective and economical solution of such type of housing, the sustainable dimension is rarely investigated. The first part explains the intergenerational relation in cohousing, the hybridity of this singular housing typology, and the system and participation of actors. The second part focuses on the research method. Based on a project-grounded research (recherche-projet) using social design tools, articulating project and sociological approaches, this contribution argues for a socio-ecosystem and enlarged definition of intergenerational homesharing. We then illustrate how sustainable relations and actions emerge from this systemic view on this housing typology.