This paper advances geographies of architecture beyond frequently studied ‘signature’ buildings by drawing attention to non‐iconic, non‐utopian, banal counterpoints – in this case, new prisons. It argues that by attending to ‘signature’ buildings, architectural geographies have overlooked the critical and underexplored circumstances and contingencies of more quotidian constructions, neglecting the mundane processes of procurement, commissioning, tendering, project management and bureaucratisation – here termed ‘architectural assembly’. Advancing scholarship in carceral geography by considering the processes and assemblages that shape (what will become) carceral spaces, it focuses on what happens before a building takes physical form. The paper draws on a major RCUK‐funded study of prison architecture to move architectural geographies more meaningfully towards a consideration of the bureaucratisation of architectural practice, as underexplored aspects of building ‘events’. It calls for geographers to pay greater attention to the banal geographies of architectural assembly, and to the banalities of production more widely.