Co-design has become a prominent vocabulary in UK urban design and planning, frequently associated with participation, inclusion, and community empowerment, especially in street-based interventions and placemaking. This article examines co-design as an urban practice embedded in everyday spatial conditions shaped by neoliberal restructuring and austerity. Methodologically, it combines a critical narrative review, used here as a selective and interpretive synthesis of relevant debates, with qualitative analysis of 23 policy and municipal documents from England, primarily London borough materials, to examine how participation is organised in practice. Through comparative reading of these materials and relevant literature, the article develops three analytical categories through which the limits of street-based co-design can be examined: temporal compression driven by funding and delivery timetables, responsibilisation through delegated maintenance and stewardship, and the datafication of lived conflict through digital engagement platforms and feedback tools. Together, these categories show how co-design operates as an interface that sustains intervention and legitimacy under constrained public capacity rather than redistributing power or resources. The article therefore reframes participation and spatial justice as questions of institutional capacity and material conditions in contemporary cities.