The objective of this article is to assess the key factors contributing to the emergence of claims in the implementation of construction projects in Poland. The study, which involved the identification and prioritization of the significance of these factors, was conducted using a mixed-method survey addressed to two stakeholder groups in the investment process: clients and contractors (a total of 242 responses were obtained). The findings indicate that the most frequent sources of claims include changes in the scope of works, design errors, and imprecise contractual provisions. The analysis revealed that construction claims most commonly pertain simultaneously to both project costs and delivery time. The results of the study contribute to a deeper understanding of the sources of contractual disputes in construction project implementation. They provide valuable insights that may support the improvement of communication between project stakeholders and serve as a foundation for developing strategies aimed at risk mitigation.