The growth of European cities in recent decades, mainly characterized by a decreasing density and a functional segregation, has tendentially increased mobility, soil consumption, urban sprawl, social disparities and infrastructural costs. Hence, most European countries have decided to aim for an urban sprawl limitation, in particular by increasing the density of built urban areas. To achieve this goal, new operations on unused urban areas are not sufficient, urban projects on existing neighbourhoods are also required. In a sustainability perspective, urban renewal goes beyond the issue of density. It has to integrate socio-cultural, economic and environmental aspects simultaneously. Based on a case study analysing the neighbourhood of "Fleurettes", in Lausanne, Switzerland, this paper shows how the diagnosis of the sustainability of an existing neighbourhood and a multi-criteria assessment of different scenarios, based on the use of the "SméO" methodology, can be an effective decision support tool for choosing an operational strategy. The results show that sustainability tends to increase with the level of intervention, especially in the field of energy consumption, but also by improving the quality of life, by promoting soft mobility, by offering green public spaces and by providing return on investments. Consequently, the research confirms the interest of the different protagonists (public administrations, private owners, investors, designers and users) to jointly explore possible synergies for the sustainable development of existing neighbourhoods.