The purpose of the paper is to describe the main spatial defects in illegal residential buildings. Three research questions are addressed: What are the main spatial defects of the dwellings? What are the main spatial defects in the relation between buildings? What is the impact of spatial defects to the minimum habitability conditions of dwellings? The defects were identified during a survey of the buildings of the Alto da Cova da Moura District (Amadora, Portugal), carried out in 2008. This district is made-up of illegal constructions, covers an area of 16.5 ha and has approximately 5,000 inhabitants. During the survey 833 buildings and 1,884 units were inspected. The main results are: a) 65% of buildings have problems with windows; b) about 20% of buildings have spatial defects in common stairways; and c) more than 50% of dwellings have spatial defects. The main conclusion is that the lack of urban planning, the confinement of the terrain and the continuous arrival of new residents have led to an overcrowded and disordered urban fabric. Furthermore, buildings are mainly self-built by residents without design and do not comply with recognized technical good practices. As a result, there is a general lack of habitability and a quick decay of buildings' condition.