person Author: Process Fellows
A Work Breakdown Structure (WBS) is a hierarchical decomposition of the total scope of work to be carried out by the project team to accomplish the project objectives and create the required deliverables. The WBS:

• Organizes and defines the entire scope of the project.
• Breaks down the project into manageable sections, called work packages.
• Serves as a foundation for estimation and planning, scheduling, budgeting, and resource allocation.
• Helps ensure that nothing is overlooked and that all deliverables are accounted for.

The so called "100% rule" is a fundamental principle in WBS design. It states that: The WBS must include 100% of the work defined by the project scope and capture all deliverables—internal, external, and interim—required to complete the project. No work should be omitted, and no extra work (outside the scope) should be included. It ensures complete coverage of the project scope and helps avoid scope creep or gaps in planning.
A frequent mistake in Work Breakdown Structure (WBS) development is the exclusive focus on development-related activities, while supporting processes such as change management, documentation, training, or deployment are omitted. Since the WBS serves as the foundation for project estimation and planning, any missing elements will lead to inaccurate budgeting and scheduling from the very beginning of the project. An incomplete WBS is one of the primary reasons why projects fail to meet their objectives, as it results in overlooked tasks, underestimated effort, and misaligned resource allocation.
A Work Breakdown Structure (WBS) does not need to be fully detailed at the very beginning of a project. Typically, a high-level structure is established during project initiation, covering the entire scope of the project. Detailed planning is usually available only for the upcoming months, depending on the project’s complexity and methodology. As the project progresses, the WBS is incrementally refined, adding more granular detail to reflect evolving requirements, deliverables, and resource needs. This progressive elaboration ensures that planning remains flexible and responsive, while still maintaining alignment with the overall project goals and timeline.