The definition of the functional unit (or functional equivalent) is one of the most sensitive aspects of a LCA when the purpose is the comparison of alternatives. The results of comparative LCA evaluations of products and technical solutions can change significantly in relation to the different functional equivalent setted with reference to various performances in use. In this paper, it is stressed the importance that the comparisons (and the functional equivalent) should be based on the specific needs of the project and on the performances to be achieved at the level of building. The functional equivalent is a critical aspect also in the LCA comparison of buildings. The need of normalization of the results of LCA of buildings is often obtained by referring at a reference unit (as the surface or the volume of the building). The paper investigates different possible assumption of reference unit that can drastically change the results of LCA.