Effective project management is crucial for the successful delivery of construction projects, ensuring they are completed on time, within budget, and to the desired quality standards. Project scope definition is at the heart of project management, which forms the basis for project planning, execution, and control. However, despite its importance, there is a lack of comprehensive investigation into project scope definition compared to other areas of management knowledge. This research paper identifies the key enablers and elements of project scope definition, as well as their relative importance. Moreover, this study develops an assessment model using the Analytic Hierarchy Process (AHP) to evaluate the completeness of the project scope definition. Data was gathered through a questionnaire survey deployed to subject matter experts specialized in building projects in Saudi Arabia. The results highlight the significance of understanding the direct impact of scope definition on project success, which emerged as the most influential enabler in facilitating scope definition. Among the top ten elements of scope definition are project financial requirements, regulatory requirements, and building site location and geotechnical data. The findings and AHP assessment model can assist project stakeholders and managers in effectively defining project scope, potentially leading to efficient management of building projects.