- Watch out
- What it costs
- Experienced readers
- Remember
- In BIMpedia
Design changes are normal; surprises about their cost aren't. Because the model knows which elements changed between revisions, the cost of a change can be worked out the day the revision arrives, not weeks later. But only if the comparison is set up properly, and someone checks what the numbers can't see.
Revision B arrives with larger south windows. The take-off reports 40.5 m² less external wall: a saving of about 11,500. The change is approved as cost-neutral. At final account, the glazing subcontractor's bill for the larger windows is about 38,000 more.
The cost manager compares revision B with the priced baseline in the model. The walls show the saving; the windows show "unchanged" because they're counted by number. Looking at the elements in 3D, the windows are visibly bigger. The report prices the extra glazing area separately and the change is approved at a net cost of about 27,000.
The change report, step by step
- 1Classify
Every measured element carries a classification code
Output: Classified model
Gate: No unclassified elements in measured categories - 2Extract
Quantities per type: count, length, area, volume
Output: Quantity table
Gate: Duplicates between models removed - 3Map
Types to cost items by classification
Output: Measured items
Gate: Every type mapped, net and gross stated - 4Price
Rates per cost item
Output: Priced items
Gate: Rates current, source stated - 5CompleteMandatory
Add what the model can’t give: preliminaries, risk, unmodelled items
Output: Estimate
Gate: Report says what came from the model and what didn’t
Each step has an output and a check. The estimate is only as good as the weakest gate.
- Save the baseline when a revision is priced and agreed.
- Open the new revision: every item shows its change against the baseline.
- Look in 3D at the changed elements, and at the ones you'd expect to change.
- Price the difference with the estimate's rates.
- Write the report: what changed, why, its cost, and what isn't included.
Every IFC element has a GlobalId that should stay the same across revisions. Tools compare revisions by matching these IDs: added, removed and changed elements. If an authoring tool regenerates IDs on export, every element looks new; check that first.
Open Quantity Take-off and 5D on the sample (sample rates are filled in). Save it as a baseline, then load Sample: revision B and choose Change since baseline.
- What does the take-off say the change costs?
- Which elements change colour in 3D, and which change can you see that the colours miss?
- What's a fair cost for the change, and how would you report it?
Key takeaways
- Every change report compares two revisions element by element, against a saved baseline.
- Price the change from the same rates as the estimate, so the numbers reconcile.
- Some changes don't show in quantities (a bigger window counted as one window); check what each quantity measures.
- Report the change with its cause, its cost and what's not included, before it's approved.
Your notes
Related tool
Quantities and costs on the 3D model: colour by cost, find what can't be measured, see each revision's change and when the money is spent.
Open Quantity Take-off and 5D →