Three essential elements of modernism consolidated through war: a centralised welfare state, a serial industrial apparatus and, often, a territorial tabula rasa. Hence, for many modernist architects and urban planners, post-war Europe became the ideal ground to put their ideas to the test. However, there is a genuine discrepancy between the proposals of the first four Congrès Internationaux d’Architecture Moderne (CIAM) and what was massively implemented throughout Europe after 1945. To explore this divergence, Brussels proves to be an interesting case study for two main reasons. First, it hosted the third CIAM in November 1930, where Victor Bourgeois presented his views on housing and cities, in line with the ideals of the time. Second, after the war, Belgium, like many Western countries, experienced a period of euphoria, during which the modernist ideology attained a sudden and broad consensus. In the capital over the following three decades new infrastructure was built, as well as housing developments that derived, at least formally, from the CIAM ideals. This article explores the gap between the ideals and the reality of modernism through a comparison on two scales: the city and housing. Bourgeois’s Grand and Nouveau Bruxelles proposals are compared to the Manhattan Plan and Etrimo’s housing developments. Understanding the gap between the ideals of modernism and its implementation may help identify characteristics of the modernist movement but also, as Lacaton-Vassal pointed out when citing Habermas, complete the “unfinished project” (Habermas, 1984) of modernism.