A good estimate is not just about getting the headline number low enough to win. It is about getting it right, so the job you win actually pays. The same handful of mistakes turn up again and again, and any one of them can cost you the work or the margin.
Below are the estimating errors we see most often, with a straightforward way to avoid each. None of them require fancy software. They require a method you follow every time and the discipline to check it.
Mistakes that quietly eat your margin
These are the errors that do not stop you winning. They just make sure the job you win loses money.
Leaving out preliminaries
Prelims are the cost of running the job rather than building it: welfare, scaffolding, skips, plant, supervision and insurances. Because they are not on the drawings, they are the easiest thing to forget.
Price them as their own named section on every tender. On most projects they are a real slice of the build value, not a rounding error, so leaving them out is the same as discounting the job.
No contingency or overheads and profit
A quote with no contingency assumes nothing will go wrong, and a quote with no overheads and profit assumes your business runs for free. Neither is true.
Carry a contingency for genuine unknowns and a clear overheads and profit line for your office costs and return. Keep them separate so neither gets quietly absorbed when you trim the price.
Out-of-date or copied rates
Reusing last year's rates, or a mate's rates, is a fast way to underprice. Material and labour costs move, and what worked on the last job may not cover this one.
Keep your own rates current from recent work and sense-check against published data such as BCIS. Treat every rate as indicative until you have measured it against the actual job in front of you.
Mistakes that cost you the job
These errors do not just shrink your margin, they make your price look wrong to the client and hand the work to someone else.
Double counting
Counting the same work twice, once in a rate and again as a separate line, inflates your price and makes you look expensive. It usually happens when measurement and pricing are rushed or split between people without a clear structure.
A tidy take-off measured in a consistent order, then priced once against each item, stops the same work appearing twice.
Vague or open-ended scope
If your quote does not say clearly what is included and excluded, the client either fears the worst and goes elsewhere, or expects everything and you end up doing free work. Both are bad outcomes.
Spell out your assumptions, your exclusions, and use a provisional sum where the scope genuinely is not fixed yet. Clear scope wins trust and protects you if things change.
How to avoid all of them
Nearly every mistake on this list comes down to method rather than skill. Price the same way every time and the gaps stop appearing.
- Work from a full measured take-off, not a number in your head.
- Use a standard template so prelims, contingency and overheads and profit are always present.
- Price against a recognised structure such as the RICS New Rules of Measurement to keep estimates ordered and comparable.
- State assumptions and exclusions clearly and use provisional sums for unfixed scope.
- Keep rates current and treat every figure as indicative until checked against the job.
When a second pair of eyes pays off
The hardest mistakes to catch are your own, especially late at night with three more tenders to price. A reviewed, independently measured estimate removes most of the risk before the quote goes out.
A professional estimating service measures the work, prices it consistently and checks it against a recognised method, so the tender you send is one you can deliver to. It is cheaper than learning the same lesson the expensive way, on site, after you have won the job.