How a construction estimate is built.
A construction estimate is four things done in order. Most bad estimates skipped one of them.
Measure
Everything starts with quantities taken off the drawings: square metres of slab, roof and wall; lineal metres of gutter and skirting; the number and size of every window and door. Not guessed from the floor area — measured, sheet by sheet.
Two things separate a measurement from a guess:
- The sheet was proven to scale first. Drawings get plotted at the wrong size all the time. Before any quantity is taken, the sheet is checked against its own printed dimensions — or better, against something the architect didn't draw, like the site width on the land title.
- Every quantity names where it came from. "Slab, 84.6 m², A-102" can be checked by anyone with the drawing. "Slab, about 85" can't.
Price
Each quantity gets a rate — what the trade actually charges per unit in that market, now. Rates come from quotes, from recent invoices, or from experience; the source matters, and an estimate should say which. A rate with no source behind it is an allowance, and it should be called one.
Allow
No drawing set is complete. Structural details, pool engineering, joinery drawings and specifications usually arrive after the costing set. For everything that cannot be measured because nothing documents it, a good estimate carries a stated allowance — a number, marked as provisional — never a blank, and never a measured-looking figure that is really a guess.
The share of the estimate that is provisional is the single most useful number in it. If it isn't stated, ask.
Check
The quantities are added up two ways. The scale is proven. The numbers that appear twice — on the summary and in the detail — are compared. The findings — the places where the drawings contradict each other, or say nothing — are listed with what they could cost, so they get resolved before the trade is let rather than argued about as a variation later.
What a good estimate contains.
- Every measured quantity, its unit, and the sheet it came from
- A grade on every line: measured, derived, estimate, or provisional
- The rates, labelled as indicative — yours to replace line by line
- The provisional share, stated as a percentage
- A findings list: what didn't line up, and what it's worth
- The basis and the exclusions, in writing
- Direct cost first — then your own margin, shown as a percentage, and the GST-inclusive figure beside it
How to check an estimate you've been given.
- Pick three quantities you can check yourself — the slab, the roof, the glazing — and measure them off the drawings. If they're within a few percent, the rest probably is too.
- Find the provisional share. If you can't, that's the finding.
- Look for a scale proof. If nobody proved the sheets, nobody knows what the quantities mean.
- Ask what wasn't measured, and why.
We do all four. You confirm the rates.
You get a bill of quantities already priced — a rate against every line and a total at the bottom — with a Your Rate column beside ours, so changing anything you would price differently moves every total with it. Plus a report showing every quantity and the sheet it came from, and a quote-request sheet per trade to send to your subbies. From $500.