The model of the dense city has become the favored answer of public policies in Swiss and more generally in European cities regarding the ecological consequences of metropolisation. However, urban renewal – in particular through the construction of « sustainable districts » - tends to induce social polarisation, thus revealing a deep contradiction between the social and ecological dimensions of urban development. Social mixity is widely promoted as a solution for reducing segregation and eviction of vulnerable populations by urban renewal projects. Yet a wide literature trend shows that many public policies aimed at social mixity did not reach their goal. Our intention here is to argue that in order to articulate the social and ecological dimensions of territorial development, we should rather think in terms of environmental justice than in terms of social mixity. This in turn entails changes both in the scale of analysis and in public action. If the concept of mixity makes sense only on the scale of neighbourhood, the concept of environmental justice invites thinking about social polarisations at least on the scale of city-center or, still better, on the scale of metropolitan area. In this paper, we first attempt to outline what the social issues related to urbanisation in Switzerland are, i.e. social polarisation and segregation, and more particularly gentrification. Secondly, we explain why public policies aimed at social mixity do not seem to constitute a satisfactory answer to the identified territorial inequalities. This leads us to justify the relevance of the concept of environmental justice and to point out how, looking at the example of the wide transformations taking place today in Lausanne – development of a sustainable district and of new transport infrastructures –, the concept operationalisation invites thinking about a new organization of priorities of public action.