"The real city, not the one portrayed in cards and in planning drawings, is the result of public and private built space and of planning regulations. But there are some perverse modes to answer land use regulations and that modes may cover a very significative part of townscape. After the '40s, Brazilian urban society grew more and more concerned with the monsters of "Urban Chaos' and "Rural Exodus'. As a reaction against that menace it developed a set of rules and regulations. It comprehends federal, state and municipal laws and regulations establishing severe limitations on urban territorial borders, urban land development and use. These laws had the effect of urban land price maximization. Indeed, through these rules government can control the supply of urban land but not the demand. At same time and during the last 50 years, migration, and therefore demand for urban space, follows increasing. To explain the maximization of urban land prices authors have used the concept of "speculative retention". However, that could explain the existence of big parcels of land inside the cities and not the scarcity of small lots. The worst effect of that situation is the "clandestine town', formed by shantytowns, slums and invasions. In São Paulo, almost 40% of people inhabits that "clandestine town". But more than 95% of that houses have water supply, 90% have electricity and 19% are linked to the municipal sewage system. Therefore, they look not so clandestine... In terms of housing typology, three general categories can be found: private, public and "clandestine". Private can be apartment buildings, groups of houses in row and "self-help" houses. Public housing is represented by apartment buildings of 4 or 5 stories, isolated houses and groups of houses in row. "Clandestine" houses are represented by shantytown houses and slums. That firs classification can be expanded to consider the several and different technologies and materials used in these types. The relevancy of that research arises from the possibility it opens for a new approach to housing programmes, to legalization and development of existing invasions and to revitalization of slum areas."