Sustainable urban planning needs the development of a systemic approach involving environmental, social and economic dimensions where the evaluation of public policies is of paramount importance for decision making. This approach requires to set up an integrated modelling platform for the computation of indicators and spatial analysis tools at different scales. In this presentation we will demonstrate first that the GIS is an ideal framework to fulfil this objective, second, that the open source paradigm is essential to face the integration of multidisciplinary packages and its use for very different professional applications, third, that the development of such software needs a specific collaborative methodology to manage the understanding and the interaction between the various stakeholders (developers, users). We shall illustrate these three points by showing why and how we have implemented these principles with the open source GIS OrbisGIS and the spatial data infrastructure CartoPolis.