The rapid growth of the older adult population and the increasing reliance on nursing homes have underscored the need to reconsider the quality of interior architecture within these environments. In such settings, spatial quality extends beyond safety, accessibility, and functional performance to encompass residents' perceptual experiences, sense of control, continuity of identity, and development of place attachment. Accordingly, the present study aims to examine the relationship between spatial adaptability and the perceived quality of interior architecture in nursing homes, with particular emphasis on the mediating role of place attachment. In terms of its objective, this study is applied research; methodologically, it adopts an analytical–explanatory approach and employs a sequential exploratory mixed-methods design. During the qualitative phase, the theoretical constructs of the study were identified through documentary content analysis of the literature on interior architecture for older adults, spatial adaptability, and environmental psychology. In the quantitative phase, a researcher-developed questionnaire was designed based on the qualitative findings, and data obtained from 186 valid questionnaires completed by experts in architecture, interior architecture, and the design of environments for older adults were analyzed. Exploratory factor analysis, together with the Kaiser–Meyer–Olkin (KMO) measure and Bartlett's test of sphericity, was employed to assess construct validity, while regression analysis and mediation analysis were conducted to test the research hypotheses. The findings indicated that spatial adaptability exerts a positive and statistically significant effect on the perceived quality of interior architecture; however, its direct explanatory power is limited. Spatial adaptability also demonstrated a strong and statistically significant positive effect on place attachment, which, in turn, significantly predicted the perceived quality of interior architecture. In the integrated model, the direct effect of spatial adaptability on the quality of interior architecture became statistically nonsignificant after the inclusion of place attachment, indicating that the observed pattern of results is consistent with the mediating role of place attachment. The findings suggest that improving the quality of interior architecture in nursing homes requires spatial adaptability to extend beyond mere physical flexibility and support residents' experiences of control, security, familiarity, meaningful memories, place attachment, and continuity of identity.