Housing is commonly discussed as the "social question of our time" in highly individualized terms. The focus is on the individual tenant struggling against rising rents, which is why the debate concentrates primarily on instruments of tenancy law. Alina Holze approaches the issue from a different angle: she understands housing as a collective phenomenon with relevance for land-use regulation. In this context, milieu protection appears as an attempt by the state to counteract undesirable developments in the housing stock. At the same time, "milieu" is not a clearly defined legal concept, but rather a normatively charged umbrella term aimed at protecting established social residential structures. While urban and social sciences describe milieus as dynamic social spaces, their legal operationalization encounters structural limits. The author therefore develops a legal concept of milieu and demonstrates why milieus deserve protection against segregation driven by gentrification. Segregated neighborhoods intensify social inequalities, increase municipal infrastructure costs, and exacerbate housing shortages in other parts of the city. Milieu protection thus addresses not only social but also functional imbalances in the housing market. Based on the welfare-state principle and constitutional duties of protection, the state intervenes through regulatory measures, in particular by means of the social preservation statute (§ 172 para. 1 sentence 1 no. 2 BauGB). However, its steering effect remains limited, even when combined with the municipal right of pre-emption. Nevertheless, milieu protection can meaningfully complement land-use planning, the promotion of new housing construction, regulations against the misuse of housing, rent-price regulation, and social welfare support, provided that these instruments are systematically integrated.