A key challenge of the 21st century is to transform society into one that features sustainable patterns of production and consumption. To achieve this, transition processes need to be designed in key areas such as housing, mobility and nutrition. The design and large-scale implementation of sustainable product service systems (PSS) is regarded a promising approach for sustainability transitions. Real-life socio-technical experiments are an important infrastructure for designing PSS in collaboration with stakeholders and users. In this paper, we argue that transdisciplinary and action research methods are required for institutionalising an experimental setup and developing PSS within such infrastructures. We present the Sustainable LivingLabs (SLL) research infrastructure and its methodology as an example of such experimental settings. It was collaboratively developed with key stakeholders in three consecutive research projects and applied to e.g. heating and space heating. We show new qualities of SLL in relation to existing LivingLabs and approaches for PSS design and present its methodological three-phase model (insight research, prototyping, field testing) of research. Our article contributes to knowledge on a methodological framework and tool-kit for PSS development in SLL with a clear focus on socio-ecological sustainability. Intermediate findings confirm the high influence of user practices on heating energy consumption and show starting points for PSS development: e.g. transformational products, home-automation combined with consulting along value chains. We hypothesise that developing PSS in userand stakeholder-integrated settings supports acceptance and diffusion and, by taking into account users' social practices of utilising novelties, reduces rebound effects caused by incorrect application.