This paper aims to show why an evaluation tool assessing the sustainability of activity systems could support farming households, particularly at the establishment stage, and how such a tool could be set up. Pluriactivity has been marginalized in France by modernization and specialization policies, despite its long-term existence and its potential to help farmers cope with sector and territory challenges. Therefore it's seem necessary to set up an operational tool that will enable agricultural advisors to support households in setting up sustainable farming projects, in pluriactivity or not. This tool should also provide project initiators with the capacity to analyse the sustainability of their project in a dynamic perspective. It should enhance support, dialogue and learning. This paper analyses three methods used to appreciate the farm sustainability and identifies not only their limits, but also their contributions to our own methodology, at the level of complex activity systems in which farming production is combined with transformation, sales or outside activities. We propose to recognize two different contributions to sustainable agriculture: a farm-focused sustainability and an extended sustainability, which means a contribution to the sustainable development at a regional scale. These theoretical elements were regularly confronted with the analysis of advisors's practices and comprehensive surveys with households in Southern France, where an analysis was carried through a partnership with researchers and local actors. It produced a tool to appraise agricultural projects, with pluriactivity or without, distinguishing farm-focused and extended sustainability.