A BIM model looks like a ready-made quantity source. Every wall, slab and door already has a size. In practice, quantities extracted from a model are only as reliable as the way the model was built, and that varies widely between projects and even between disciplines on the same project.
Before relying on model quantities, four questions are worth asking.
1. Is the model detailed enough for the quantity you need? A wall modelled as a single generic element cannot give separate quantities for blockwork, plaster and paint. If the measurement needs layers, the model has to carry them.
2. Are elements classified consistently? Quantities are grouped by element type, family or parameter. If similar items are modelled under different types, or different items share one, the extracted totals will be grouped wrongly even when every individual size is correct.
3. Does the model match the drawings being issued? Drawings are often finished in 2D after the model stops being updated. If the issued drawings and the model differ, the BOQ has to follow the contractual documents, and the model becomes a check rather than the source.
4. What is not modelled at all? Finishes, small fixtures, temporary works and many builders' work items are often absent. A model-based take-off needs a clear list of what was measured manually alongside it.
When the answers are favourable, model-based quantities are fast, repeatable and easy to update as the design changes. When they are not, the model still adds value as a cross-check: comparing 2D quantities against model quantities is one of the quickest ways to find coordination gaps.
The practical approach is to decide, element by element, where the model is the source and where it is the check. BIM then strengthens the measurement instead of replacing judgement.