This article examines the history of, and specialist design principles behind, two Admission Units that were added to two existing mental health hospitals in post-war England: Fair Mile Hospital, in Cholsey, near Wallingford, Berkshire (now Oxfordshire) and Fulbourn Hospital, near Cambridge, Cambridgeshire, completed in 1956 and 1964 respectively. Their commission and realisation fell within a period of critical psychiatric reforms, some of which first emerged in the first half of the twentieth century with further developments peaking after the Second World War. Although the two buildings preceded subsequent interdisciplinary advancements that specifically fed into the correlation between buildings and healthcare, namely, environmental psychology and evidence-based design, they emerged at a time when prominent hospital architecture studies were embracing a research culture and aimed at merging an interdisciplinary approach with modernist architectural principles. Yet these two buildings, and numerous other similar units, have been virtually absent from existing scholarship, both general architectural historiography and studies of specialist healthcare architecture. The study here uses literature review of architectural historiography, historic policy and guidance documents, and archival material, as well as review of graphic material comprising photographs, architectural drawings and diagrams. The discussion opens with a brief overview of the general policy and psychiatric context within which the two units were commissioned and erected. This is then followed by a closer inspection of the two buildings, which brings to light both similarities and differences. Discussed in parallel to principles outlined in healthcare architecture guidance published at the time, this comparison highlights the complex and nuanced associations between architectural design and mental healthcare, as well as the way these evolved in this particular time period. Specifically, by looking closely at national and international guidance on healthcare architecture developed by the Nuffield Provincial Hospitals Trust, the World Health Organization and the Ministry of Health, key shifts in the role that architecture could play beyond a medical model of mental healthcare and in support of a psychosocial approach are discerned. Finally, the article also makes the case for further research that broadens and deepens studies of the post-war history of this field as well as comparisons both to the inter-war and to the contemporary periods.