Ambitious building retrofits to improve energy performance are often hardly justified by energy savings only. Indeed the return on investment to divide energy consumption of a building by two is more than 25 years, which often discourage investors. Energy efficiency thus needs to be considered differently to be economically justified. That is why a new methodology called DECADIESE has been developed by a consortium of major companies from the French building sector and academic partners. Broader than an energy efficiency focus only, DECADIESE aims at capturing the sustainable value of a building with an original focus beyond the scope of classical analytic methods, by extending the perimeter of associated stakeholders in order to allow ambitious building projects. DECADIESE considers an extended value of a building by incorporating economic, social and environmental aspects through externalities elicitation and integration with a multi-stakeholders point of view, but also by recentering the value created by a building around the benefits brought to its users through a functional approach. The methodology is built around a structured process involving multiple stakeholders (such as building owner, building owner assistant, engineering contractor, potential users and neighbours…) that contribute to feed the different modules of an experimental tool, finally aggregated in cost-benefits indicators. This paper focuses on the functional part of this model. On the one hand, buildings costs are first broken down into usage functions following a process inspired from value engineering standards. Usage functions costs offer a new vision of costs breakdown for a building, which highlights possible mismatches between functional objectives for users and associated amounts of money. On the other hand, the functional performance of the building is assessed thanks to a grid composed of 95 qualitative and quantitative criteria. Those elementary scores are then aggregated in 7 usage functions scores thanks to a fuzzy logic model based on building experts rules. Once those elements identified, DECADIESE proposes to a project owner a new vision of its project and the ability to compare building variants. DECADIESE has been experimentally applied on several case studies. Results show a good relevance of results and reveal a high interest of building sector actors. Next steps are underway to make the methodology more reliable and usable by building sector actors.