By 2020, the manufacturing and construction industries sector is responsible for 16.3 percent of Europe’s greenhouse gas emissions, third after the energy industries sector (31.4%) and transportation sector (29.0%). The built heritage is one of the first targets for mitigation and improvement of current environmental conditions. Carl Elefante’s now famous statement “The Greenest Building Is... One That Is Already Built” wide the aspiration of the green concept to building design not only to new construction but also to the existing building stock. The purpose of the hypothetical intervention would aim at improving the three dimensions of sustainability - environmental, economic, and social - as applied to the building artifact. The dimensions of sustainability related with the concept of green building, that aims to implement user health through the design of healthy indoor environments. As a result of providing the definitions of Green Building design mainly found in the literature, the first approach of the paper is a critical reading of three case studies in the field of sustainable building, the criteria used in ex-ante evaluation, comparing their results, and providing a general procedural a logical framework to understand the initial part of the design process approaching to sustainability.