The ratio of public housing to population has moved in highly contrasting patterns in French urban units. Given the economic and social consequences of the nationwide distribution of public housing, it is necessary to understand the mechanisms that explain the variations in public-housing supply. Our article focuses on two of the main determinants of the changes in the public-housing-to-population ratio. First, we identify a strong, negative link between the change in the fragmentation among municipalities within an urban unit and the change in the supply of public housing measured against the local population. The most segmented urban units, i.e., those whose population is distributed across many municipalities, display a form of governance in which decisions are relatively more decentralized. Municipal borders thus make it possible to differentiate public-housing supply between municipalities within an urban unit. We show that urban units whose population has become more fragmented have built less public housing than urban units that have remained more integrated. Second, the durability of housing means that public-housing supply policies persist over time. It is thus hard for supply to adjust rapidly to economic shocks. As a result, if they do not destroy public housing, municipalities in decline may experience an “automatic” increase in the ratio of public housing to population. We show that, in declining urban units, the ratio effectively rose faster in 1975-1999 than in other periods. The differences in public-housing supply between urban units may thus endure if the public-housing stock does not decrease when the population declines.