In the analysis of the informal city, the description of housing units is usually more relevant than the study methods. While there are precise methodologies for the metropolitan and neighbourhood scales, the approaches for the study of housing units, when confronted with reality, are insufficient for reaching the complexity of the problem. The aim of the study is to identify and synthetize the used methodologies for the study of informal housing, from the second half of the 20th century. We applied PRISMA methodology for the systematic revision of available literature, which allows the identification of relevant papers based on search terms. We filtered systematically a total of 203 indexed papers, and we obtained a final selection of 19 papers. We analysed the approach methodologies in order to identify the indicators used by the authors for the analysis of the informal house. The set of indicators was organised in six aspects and 26 variables, in a synthesis matrix of existing studies, which reveals the different possible approaches. In this study, we observe the complexity of the phenomenon through multidisciplinary aspects, and we propose a theoretical tool for future research.