Communities around the world share a common challenge: how to develop existing environments and plan new communities that are able to combine social and ecological sustainability of the environment. This challenge was topical also in the City Plan process of the city of Helsinki between 2013 and 2016. Among the tools of participatory urban planning, the city of Helsinki applied an online PPGIS (public participation GIS) method. This chapter describes this real-life participatory urban planning case and a preceding research project called “Urban Happiness”. Also, a theoretical framework concerning social sustainability is discussed. Both the research project and the participatory planning process applied a place-based approach that allow a context sensitive analysis of the prerequisites of socially sustainable urban environment that focuses on the individual resident’s perspective. The empirical datasets that were collected for the research project (n=3100) and the city planning project (n=3900) show that it is possible to produce diagnostic knowledge combining “soft” and “hard” localized knowledge and to realize large-scale public participation in a master plan level. The effective use of this knowledge is a crucial next step towards the development of socially sustainable urban settings.