A Work Breakdown Structure is the hierarchical breakdown of a project into progressively smaller deliverables and work packages, ending at the level where each package can be estimated, scheduled, and assigned to a single party. The top of the hierarchy is the project itself; intermediate levels group related scope (sitework, structure, envelope, MEP, finishes); the bottom level is the work package, the unit at which budgets and schedules are managed. WBS structure typically follows CSI MasterFormat divisions or a project-specific custom hierarchy.
WBS is the foundation for both estimating and scheduling. The estimator builds the cost estimate by pricing each work package. The scheduler builds the CPM schedule by sequencing the same work packages with durations and logic. The contract's schedule of values mirrors the WBS so that pay applications track to the same structure used for budgeting and execution. Inconsistent WBS across estimating, scheduling, and billing is one of the most common reasons project tracking falls apart on commercial work.