In building construction projects there are 3 factors that are used to measure project performance, one of which is cost. Efforts that can be made to minimize the cost overrun are knowing the causative factors and knowing how to mitigate them. The purpose of this study is to identify the causative factors, determine the dominant factors causing cost overrun, and efforts to mitigate them. The methodology in this research is desk research or research that uses relevant secondary data analysis, such as previous research with similar problems. The results of this study were 65 causative factors divided into 10 groups. The dominant factor causing cost overruns includes material, implementation and employment relationships, project documents, and project finance groups, one of them is fluctuations in material prices. Mitigation efforts have been obtained for the dominant factor, namely quality control in the field must be good, selection of professional workforce, and building good relations between parties.