Little is known about the experiences of male patients in secure mental health and intellectual disability units during environmental transition. We interviewed patients before (n = 9) and after (n = 8) a side-to-side security transition from medium-secure wards in an older building to new wards in a purpose-built building. We inquired about transitional experiences in general and about this transition specifically. We examined interview transcripts and field notes using thematic analysis, and collated routine outcome data to gauge whether transition had obvious positive or negative effects. Qualitative analysis indicated three major themes (information, transition, and behaviour) and five overlapping subthemes (positive information sharing and consultation, concerns and anxieties about lack of information, life change and opportunity, home and sense of belonging, and potential conflict). Outcome data indicated little obvious change between first and second interviews. Expressed concerns of patients about transition were largely about tangible and practical issues, including changes to rules, including smoking and prohibited items. The results highlight the need for the development of supportive patient-inclusion strategies, consistent and transparent communication processes, and a published timeframe during the planning and implementation phases of all types of transitional moves, including the side-to-side transfer of residents between accommodation buildings.