Urban sustainability interventions must always contend with the built environment––the historically accumulated materialization of previous rounds of city building. In this article we argue for a historicized perspective on urban sustainability that allows us to consider how ideals of the past, materialized in the urban built environment, constrain current sustainability interventions. Drawing on the policy mobility literature, science and technology studies, and assemblage thinking, we examine the recursive relationship between mobile ideas on the one hand and urban materiality on the other. Empirically we analyse the historical development of Forus Industrial Park in Stavanger, Norway, and current attempts to transform this into a compact, ‘post-fossil’ space. We focus on two phases: the late 1960s, following the discovery of oil, when Forus started developing into the core oil cluster in Norway, drawing on ideals from Houston and Milton Keynes; and the current preoccupation with sustainable mobility and density, drawing on ideals from compact European cities like Copenhagen. We show how the inertia of the built environment exercises a form of distributed agency constraining interventions towards sustainable urbanism. Therefore, we argue, truly powerful interventions in urban sustainability are those that effectively reconfigure and retrofit, and reassemble existing materialities into new formations.