Using quantities from the model, mapped to a cost structure and priced with rates, to produce and update estimates, and knowing which parts of the estimate the model can't provide.
Model-based estimating takes quantities from the model, maps them to the items of a cost plan or bill, and prices them with rates. When the design changes, the quantities update and so does the estimate. It is often called 5D BIM. The model provides the measured quantities; the estimator still provides judgement, rates, the items that aren't modelled and the risk.
A model knows "Basic Wall: EXT-200 Blockwork + 100 Insulation + Brick". A cost plan knows "2.5.1 External walls". Someone has to say which model types go into which cost items, and how they are measured. Teams make this mapping repeatable by giving elements a classification code (Uniclass, UniFormat or an internal code) and a type name that tells the estimator what is inside.
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