In this article, the sustainability assessment of land use efficiency of buildings is subject to a critical study. Several building sustainability assessment (BSA) tools were analyzed, finding that the impacts of this criterion are not correctly assessed in current methodologies and some even ignore its importance. Some existing assessment methods and corresponding indexes were used in four virtual buildings to evaluate the effectiveness of each calculation method. It was concluded that the existing methods do not provide correct assessments and can promote unsustainable practices. It was also proved that it is necessary to include the implantation index, spatial efficiency, and occupational efficiency for correct assessments complementary to the use of the gross floor area index or development density. Due to difficulties in the weighting of the four complementary indexes, a single index was developed to perform the calculations using five main variables. The new index was tested in seven virtual buildings for internal validation and to two real buildings, providing good assessment results regarding the expected performance of buildings and proving to be better than other indexes that are currently used in BSA tools to assess land use efficiency. Considerations are also made for the development of regional benchmarks for the new index and the possibility ofusing it in regulations at a municipal and national scale to promote land use efficiency.