A growing majority of people living in Residential Care Facilities (RCFs) for older people have dementia. Yet the implementation of evidence-based Dementia Design Principles (DDPs), known to reduce symptoms and improve wellbeing, remains limited. This paper reports on the development and application of Plan-EAT, a floorplan-based method of assessing dementia design quality in RCFs. Through the analysis of 34 published architectural exemplars, the method identifies strengths and weaknesses in the layouts of residential care buildings, across 39 assessment criteria organized under 9 DDPs. The paper concludes that the Plan-EAT could benefit architectural practice by providing an evidence-based means of assessing layout planning quality, in both existing cases and emerging RCF design proposals. The findings also highlight the need for care, where reliance is placed on published case studies, to inform the design of dementia supportive environments.