The classification of informal housing constitutes a significant topic to historicize the conceptualization of habitability. This article examines some categories used by censuses and sociological surveys in Italy, with the aim of highlighting how the identification of urban informality changed in relation to the transformation of political and ideological contexts. For a long period, public concern was limited to the issue of unhealthy dwellings in the most degraded buildings within the official city. Only in the 1950’s, the recognition of the existence of informality assumed a more metropolitan dimension, as a phenomenon connected to the development of spontaneous settlements. Within this transformation, which could be summarized as a shift from insalubrious housing to irregular neighbourhoods, it is also possible to read an evolution of the concept of habitability, which tended to move from a strictly hygienic sphere towards a community model.