How to Check a Construction Estimate Before You Rely on It

Why an estimate is only as good as its paper trail

A number on its own tells you nothing about how it was derived. Two estimates for the same project can both look plausible and land $200,000 apart, because one was measured off the drawings and the other was benchmarked off a rough rate per square metre. Before you rely on any construction estimate to secure funding, brief a builder, or set a client's expectations, it's worth knowing what to check.

Ask for the basis of estimate

Every credible estimate should come with a basis of estimate: a short statement of what drawings, specifications and revision the pricing was measured against, and the estimate class (see Class 3 vs a tender-stage estimate) it was prepared to. If an estimator can't tell you which drawing set and revision the number is traceable to, the number isn't traceable, full stop.

Check the assumptions and exclusions register

No estimate covers everything, and that's fine, provided the gaps are written down. An assumptions and exclusions register should state plainly what's included, structure, external walls, standard finishes, and what's excluded: landscaping, council contributions, geotechnical or contamination unknowns, and whether GST is in or out. An estimate with no exclusions listed almost always has hidden exclusions; someone just didn't write them down, and you find out on site.

Know what's measured and what's an allowance

A good estimate distinguishes measured items (quantities taken directly off the drawings, at firm rates) from allowances (a placeholder dollar figure for scope that isn't fully designed yet). Ask which line items are which. Where you see Provisional Sums and PC items, ask what they cover and how they were set: a PS sum for earthworks based on a site with no geotechnical information is a bigger risk than one supported by a site classification report. An estimate that's mostly allowances with a few measured items dressed up as firm numbers isn't a properly measured estimate, whatever it's labelled.

Questions to ask any estimator

• What drawing set and revision was this measured against?
• Which items are measured quantities and which are allowances?
• What's explicitly excluded, and is GST in or out?
• Where are the Provisional Sums, and what are they based on?

At HYKSOS every estimate and takeoff ships with drawing references, a stated basis of estimate, and an assumptions and exclusions register as standard, not as an optional extra. That's the traceability a number needs before anyone should rely on it.