English: Delays to contractors’ progress, often resulting in time and cost overruns, are a major source of claims and disputes in the construction industry. The assessment of extension of time (EOT) claims as part of a construction project can have far-reaching consequences for the financial success of the project. The proper and transparent assessment of EOT claims is, therefore, an essential component in the success of any project. In this article, an action-research approach, a very specific qualitative approach, was followed to develop a user-friendly guideline to help practitioners navigate this potential minefield of complexities in the process of the assessment of EOT claims. Focus groups, consisting of industry practitioners, with specialist knowledge in construction contracts, contributed to the development of the decision-support frameworks, and ultimately to the findings. The iterative process followed assisted in producing a tool that can be used in practice as a guideline for the analysis of EOT claims when using the Principal Building Agreement of the Joint Building Contracts Committee (JBCC).