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.
At a glance
| Who uses it | Cost managers, quantity surveyors, contractors' estimators, clients' finance teams |
| Chain | Model elements → quantities → cost items (by a cost structure) → rates → estimate |
| Cost structures | Elemental (for example NRM 1 groups, UniFormat) for early estimates; trade or work sections (NRM 2, MasterFormat) for tender and contract |
| Grows with the design | Early: areas and counts per element group. Later: detailed quantities per type |
| Never fully in the model | Preliminaries, overheads and profit, risk, inflation, temporary works, unmodelled items |
The mapping is the real work
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.
| Model type | Classification | Quantity used | Cost item | Rate (example) |
|---|---|---|---|---|
| EXT-200 Block+Ins+Brick | Ss_25_10_30 (wall system) | Net area, m² | External walls, cavity, brick outer leaf | 285 / m² |
| INT-100 Stud+2xPB | Ss_25_30_20 | Net area, m² | Internal partitions, stud | 68 / m² |
| Door D1 900×2100 | Pr_30_59 | Count | Internal doors, single | 640 / nr |
| Slab 250 RC | Ss_30_12 | Volume, m³ | Upper floors, RC | 210 / m³ |
The rates above are illustrations, not prices. Real rates depend on country, date, procurement and market.
Early and late estimates
- Concept: the model gives gross floor areas, wall-to-floor ratios, façade areas, counts of rooms. Estimates use cost per m² and elemental benchmarks.
- Developed design: quantities per element group (all external walls, all partitions) priced with composite rates.
- Technical design and tender: quantities per type, measured to the rules the contract uses, priced item by item.
Worked example
Between two design revisions, the architect changes the façade from 40% to 55% glazing. The estimator re-reads the model: external wall area drops by 310 m², curtain wall rises by 310 m². At the illustration rates above (wall 285, curtain wall 950 per m²), the change adds roughly 310 × (950 − 285) = about 206,000 to the estimate, in an hour, with the quantities traceable to the revision. Without the model, it would have been a re-measure from drawings, and probably a day later.
What the model can't tell the estimator
- How the work will actually be built (method, plant, temporary works), which drives cost as much as quantity.
- Items below the modelled detail: fixings, sealants, small fittings.
- Whether two disciplines modelled the same thing twice (both architect and structure modelling the slab).
- Whether the model's quantities follow the measurement rules (a wall drawn through the floor zone is not the area a measurement rule asks for).
Common mistakes
- Pricing schedule totals without checking scope and duplicates.
- Mapping by type name text instead of a classification property, so new types fall through the gaps.
- Mixing net and gross quantities.
- Presenting a 5D estimate as complete when it covers only modelled work.
Where to read more
National measurement and cost-planning rules (in the UK, the RICS New Rules of Measurement suite; internationally, ICMS; in North America, UniFormat and MasterFormat) define structure and measurement. RICS and similar bodies publish guidance on BIM for cost management.