A breakdown that adds up
Indent to nest. Codes generate themselves, estimates roll up from the bottom, and a number typed against a parent is ignored rather than counted twice.
A numbered breakdown that adds up
Type the structure with indentation. Codes are generated, estimates roll up from the work packages, and a figure typed against a parent is ignored rather than double-counted.
The structure
One item per line. Indent with two spaces (or a tab) for each level down. Add an estimate after a pipe: Detailed design | 8.
Numbered breakdown
How the roll-up works
Estimates belong on the work packages at the bottom. Anything with children below it gets its figure from them, by addition. A number typed against a parent is reported and then ignored.
That rule prevents the classic double-count: a parent estimated at 40, with children adding to 40, giving a total of 80. It is easy to do and hard to spot once a structure is more than a screen tall.
Codes follow position, not identity. Insert a line and everything below renumbers, exactly as it should. If you need a code that never moves, that is a different thing from a WBS code and should live in its own column.
An over-indent is repaired rather than obeyed. Jumping three levels in one line is a typo far more often than it is a structure, so the item is placed one level below its parent.
Deliverables, not activities
The most common way a WBS goes wrong is being written as a list of things people will do rather than things that will exist. "Design", "Build", "Test" is a process. "Approved space plan", "Fitted-out floor", "Signed-off network" are deliverables, and they are testable in a way that verbs are not.
The practical difference shows up at the end. You can tell whether a deliverable exists. Whether "design" is finished is a matter of opinion, and that opinion is usually contested at the least convenient moment.
The 100% rule
The children of any item should add up to all of it and nothing more. If a parent has three children covering 80% of the work, the missing 20% does not appear anywhere in your plan, will not be estimated, will not be scheduled, and will still have to be done.
That is worth checking deliberately at each level, because the omission is invisible — the structure looks complete, and nothing in it points at the gap.
How far down to go
Far enough that a work package can be estimated with some confidence and owned by one person, and no further. Decomposing a two-day task into six twenty-minute steps adds administration and no information.
A rough test: if you cannot estimate it, it is too big; if tracking it costs more than doing it, it is too small.