The theme of this dissertation is the study of housing in Lisbon's Baixa Pombalina blocks of flats. It aims to understand the spatial organisation of houses and to study the ways of qualified intervention, thus seeking possible guidelines for future projects. A priority of intervention in the Lisbon's Baixa is the qualification of housing, which would allow for a more diversified and effective occupation. The research is divided into three parts. First, we looked into the evolution of anti-seismic structure in wood and of colective urban housing. Secondly, the specific organisation of housing was studied, through the analysis of first floor living plans, which were originally collected by Jorge Mascarenhas (1996). This research was further developed through the in-depth analysis of qualified interventions. Twelve cases of intervened buildings or flats were studied, most of which were designed by architects. Whenever possible the organisation of the flats, the structure and the function of the rooms were compared before and after the interventions. Finally, with reference to the Space Syntax model, the functional and spatial organisation of the flats was graphically represented in a simplified way, by means of justified graphs, which make it possible to compare the relations between convex spaces.