Since the lifestyle of societies change in time, which results in different needs and desires, monumental buildings lose their original functions. Preservation and continuous utilization of monumental buildings is possible if they are given new functions while their value and specific characteristics are preserved, where a balance is guaranteed between the new function and the original space. With the new form of utilization, it is critical for the ability of the space to respond to the needs of the new function, and for the continuity of the new function and thus sustainability of the building. In order for preservation to be sustainable, the building should be able to respond optimally to the needs of its new users and requirements of its new functions; thus the adaptation between the new function and the monumental building should be ensured. In order for identification of such adaptation level, the spatial and operational requirements of the new function should be set forth and an evaluation should be done on whether the monumental building is suitable for the new function. The evaluation results would provide data for the following practices and help selection of the new function. In this study, the adaptation level of the Sahabiye Madrasa of Kayseri, which is being reutilized under the name, "Book Bazaar", to its new function is investigated with the spatial analysis method. The study suggests that, although the madrasa had an open patio and single-floor plan, its original plan, architectural composition and spatial characteristics of the madrasa were modified during its utilization as the Book Bazaar, and the interventions to the original spaces within the building damaged the historical identity, esthetics, documental value and authenticity of the building.