The research is about the assessment of the sustainability level of residential buildings in the city of Lima (Peru) during the occupancy stage of their life cycle, thus generating a proposal for a national sustainable certification methodology adapted to the real situation of the country (main problems, regulations, standards, among others), which allows the most appropriate improvement plans to be proposed. First, the diagnosis of the sustainability characteristics that need to be assessed in buildings in Lima is made. Then, the assessment and scoring method of a situation with such characteristics is presented, which shall be part of the sustainable certification proposal called GREEN UNI. Finally, the certification is applied to a sample of 20 buildings in different areas of Lima, thus recognizing the sustainable situation of buildings in the city.