This study reviews the existing provisions for variations in the Public Sector Standard Conditions of Contract (PSSCOC) for Construction Works and identifies the most frequent and expensive root causes of consultant-related variations in school building projects in Singapore. The case study approach was adopted. Information for the study was obtained from source documents of 80 school building projects completed, and through personal interviews and in-depth discussions with the professionals with a government agency, as well as the consultants and the contractors who were involved in the school projects. The 20 causes of variations from the consultant-related variations (CRV) group were selected for this study. The results suggested that the upgrading projects experienced almost double the variations compared to new projects. The non-compliance of design with government regulations, design discrepancies, change in design by consultant were considered as the top five most frequent and expensive root causes of CRV in school building projects. Design consultants should pay close attention to the more frequent and expensive causes of variations in the context of clauses 19 and 20 of the PSSCOC. While the provisions for managing and valuing variations judiciously are already well provided for in the PSSCOC, consultants should pay heed to some of the more common and costly CRV identified in this study and avoid their corresponding pitfalls. This should then path the way for smoother progress and better working relationships among all parties in the building project.