Sustainability assessment tools are currently influencing sustainability strategies and activities in higher educational institutions. Thus, in this paper a short overview of sustainability in universities will be provided, which will then be followed by a study of different existing approaches to assess this issue. The paper will also include previous analyses that theoretically compare these tools. A practical case study in which different tools are applied is the main goal of this article. A sustainability diagnosis and a proposal to analyze the results in order to create an action plan, based on cost-efficient measures, will be developed considering different tool's approaches. Advantages and differences between tools are highlighted according to the results. Finally, this article will conclude that an absolute improvement between 20% and 40% in the overall sustainability score can be reached in the medium term.