This article builds on recent interventions into the study of planetary urbanization that call for greater interaction with the multiple social struggles and standpoints that embed this process. To do so, we advocate for academic engagement between planetary urbanization and the concept of the ‘construction state’. This is a term used in Japan and South Korea to describe an alliance between development corporations and the state, one whose expansionary logic accords with the notions of planetary and extended urbanization and acts as a co-constitutive, regional driver of it. To better situate this concept, and to highlight the social struggles that inform it, we examine the politics of greenbelt deregulation and the expanding real estate-led urbanization in the Seoul Metropolitan Region in recent decades. We show how the construction state and its supply-centrism played a key part in the ‘explosive’ process of greenbelt development and examine the dynamics of the struggles that activists waged against it. By doing so, we contribute to ongoing debates about planetary urbanization as an open totality of processes, stress the importance of a situated approach that foregrounds the diverse practices and struggles that shape it and encourage communication between critical standpoints.