Historic buildings that undergo adaptive reuse frequently experience gradual functional decline over time when they fail to adequately accommodate new demands a process that ultimately results in functional incompatibility and premature physical deterioration. This problem typically stems from prserveation decisions made without a rigorous assessment of a house's adaptive capacity. The Mashrouteh House stands as a prominent example of this phenomenon: despite its considerable historical significance, the house has suffered a marked decline in functional appeal due to poor management and a change of use carried out without prior feasibility study. The central research question is twofold: to what extent does functional incompatibility affect the Mashrouteh House, and which criteria are most urgently in need of improvement? The primary objective of this study is to evaluate the functional flexibility and adaptability of the Mashrouteh House in its current use through the application of the FLEX 4.0 model, and to propose adaptive intervention strategies accordingly. A mixed qualitative-quantitative methodology was employed, utilizing the standardized FLEX 4.0 questionnaire comprising 32 items across five principal criteria site, structure, skin, services, and space plan to collect data from 32 participants, including 15 conservation specialists and 17 end users. Findings revealed that the Constitutional House achieved a score of 168 out of 512, placing it at Level 2 (very low flexibility), with site, structure, and services identified as the criteria requiring the most significant improvement. Gap analysis between expert and user perspectives indicated that supply exceeds demand across 9 indicators, while demand exceeds supply in 5 indicators. The outcomes of this research can contribute to enhancing rehabilitation quality with minimal cost and time investment, and may be incorporated into the cultural heritage database of East Azerbaijan Province to inform and improve the functional performance of comparable historic buildings.