Cost management is one of the most important measures of project success throughout the project’s lifecycle and is of high concern to those involved in the building rehabilitation project. The purpose of this research is to study the factors that affect the cost management of building rehabilitation projects and to find the relative importance index (RII) of each factor. The research methodology includes case studies, brainstorming, and expert interviews followed by questionnaire. The questionnaire includes 57 factors that are divided into six main groups: design and bill of quantities, planning and contract management, building characteristics, economic and financial, safety and environment, and project management. The results indicate that the most influential factors are the required quality standards with RII (84.9%), site visit to the building before the design process and preparation of the bill of quantity with RII (83.9%), availability of materials in the local market with RII (83.4%), number of floors in the building with RII (85.5%), building or project location with RII (85.4%), building condition with RII (85%), labor costs RII (86.7%) and currency exchange rate fluctuations (RII 87.1%). Overall, the study emphasizes the importance of identifying and addressing the key factors affecting cost management in building rehabilitation projects.