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Chapter 3 · Lesson 2 of 3Topic: Cost, time, contracts

Estimating and quantity surveying

Estimators and quantity surveyors turn designs into money: measure quantities, apply rates, build cost plans and value change. They trust model quantities only when classification and measurement rules are clear, which is where you come in.

With Kai, site manager2 min read3 quiz questions
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  • Watch out
  • What it costs
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Kaisite manager
Even if you do this every day, watch for this: Model quantities follow how the model was built, not the measurement rules a QS works to. A wall modelled through two floors, or a slab that includes the beam, measures differently. Agree what the model measures and say so on every take-off.

Estimators and quantity surveyors (QS, also called cost managers) put prices on designs. Estimators work out what something will cost to build; QSs manage cost through the whole project: cost plans, tender pricing, monthly valuations, and the value of changes. Model data can save them weeks, if they can trust it.

Basics
ActivityWhenWhat the QS needs
Cost planDesign stagesAreas, element quantities by classification
Tender pricingBefore contractMeasured quantities, specifications
ValuationsMonthly on siteWork done, materials on site
ChangeWhenever the design changesWhat changed, by how much

Making model quantities trustworthy

  • Classify elements consistently, so they group like cost items.
  • Agree what the model measures: net or gross areas, how walls through floors are split, whether slabs include beams.
  • Flag what isn't modelled: preliminaries, temporary works, finishes below the modelled detail.
  • Compare revisions to give objective change quantities.
Story · The 20% overrun that wasn't
How it goes wrong

The QS takes wall areas from the model at stage 3 and finds 20% more than the stage 2 cost plan. The client panics. Weeks later, the cause turns out to be walls modelled through two storeys at stage 3, double-counting their area against the measurement rules.

How it goes right

The take-off states how walls are modelled and measured, and flags elements without classification or quantities. The QS adjusts the two-storey walls, and the real change (3%) is reported the same week.

In BIMpediaOpen the sample IFC file and run the five-minute check: header, structure, mapping problems and required properties.
Kaisite manager
Your turn. Give it about 10 min and work it out on your own before you open the model answer: that is where it sticks.
Exercise: Make a take-off defensible 10 min

You're sending model quantities to the QS for the stage 4 cost check. Write the four notes you'd put at the top of the take-off.

Key takeaways

  • Estimators price designs; quantity surveyors (cost managers) manage cost through the project: cost plans, tender pricing, valuations and change.
  • They work to measurement rules (for example NRM in the UK) that may differ from how models are built.
  • Classification lets model elements map to cost items; without it, quantities can't be priced reliably.
  • Model comparison between revisions gives fast, objective change quantities.

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 →

Check yourself

3 questions · 2 right to complete
  1. 1. Why might a QS not trust a quantity straight from the model?
  2. 2. What lets model elements be mapped reliably to cost items?
  3. 3. The design changes between revision B and C. What's the fastest objective way to see the quantity change?
0 of 3 answered
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