On a process plant, a unit rate against a quantity gets you close. On a berth, a causeway or a deep excavation next to a live rail line, it gets you a number that has nothing to do with how the job will be built. The cost is in the method, so the estimate has to start there.
This is the single biggest divide in estimating practice, and it is invisible on the summary page. Two estimates can carry the same line, the same quantity and the same total, and one of them is built on how the work will actually be done while the other is built on hope.
Where unit rates stop working
Unit rates earn their keep on repetitive, market-tested work: paving, standard formwork, straightforward earthworks with open access. The rate has method baked into it because the method barely changes from job to job.
They stop working the moment the method stops being standard. Constrained access, marine plant, staged construction around live operations, work inside a shutdown window: on these jobs the cost is driven by time and by the equipment spread, not by the quantity. A crane on a barge costs the same per shift whether it drives twelve piles or six. When the window, the access or the sequence cuts productivity in half, the quantity has not changed, the drawing has not changed, and the cost has doubled. A unit rate borrowed from the last job carries the last job’s method into a job that will not be built that way.
Build the plan first
Before anything is priced, we write a short method statement for each major element: what equipment, from where, in what sequence, with what constraints. A sheet pile wall driven from land costs one thing; the same wall driven from a jack-up in a tidal window costs another. The drawing is identical.
The method statement answers five questions, and every one of them moves money.
- Access and laydown: where does the crane stand, and for how long?
- Sequence: what has to finish before this can start?
- Windows: tides, fish windows, weather, shutdowns, traffic management hours.
- Crew and equipment spread: the actual people and plant on site each day.
- Duration: from the spread and the productivity, not from the schedule someone hopes for.
One wall, two jobs
Take a sheet pile wall, call it 240 piles, and keep the numbers illustrative so the mechanics stay visible.
Driven from land, the plan is a crawler crane with a vibratory hammer and a crew of five, working open ground with good laydown behind the wall. Call productivity twelve piles per shift, so twenty shifts. Crew and plant together, call it 18,000 dollars per shift. Installation lands around 360,000 dollars, plus the permanent materials, which cost the same in both versions of this story.
Now put the same wall in the water. The plan becomes a jack-up platform, a larger crane, a support boat, and a tidal window that limits productive driving to part of each shift. Productivity drops to six piles per shift, so forty shifts. The marine spread runs, call it 34,000 dollars per shift, and the jack-up costs 250,000 dollars to mobilise before the first pile is touched. Installation is now about 1.6 million dollars.
Same drawing. Same 240 piles. More than four times the installation cost, and every dollar of the difference lives in the method: the spread, the productivity and the mobilisation. An estimator pricing this wall from a unit rate has already decided which job it is, whether they know it or not.
Then price the plan
Once the spread and duration exist, most of the cost is arithmetic: crew cost per shift times shifts, plant cost per shift times shifts, plus the permanent materials. The productivity assumptions are explicit, written next to the calculation, and can be challenged one at a time. A reviewer can ask why twelve piles and not fifteen, and the answer is about ground conditions and hammer size, not about a rate from a book.
The duration that falls out of the plan does one more job: it prices the indirects. Site overheads are mostly a function of time, so an estimate whose durations come from crew spreads and productivity can price a schedule change in an afternoon. An estimate built on unit rates cannot price it at all.
Two identical drawings can be two different jobs. The estimate has to know which one it is pricing.
Where this goes wrong
The borrowed rate. A unit rate from a database or an old bid arrives carrying its original method, access and market. Nobody asks what job it came from, and the new job inherits assumptions that were never true here.
The hopeful duration. The estimate takes its durations from the schedule instead of building them from the spread and the productivity. When the schedule was drawn to hit a date rather than to reflect the work, the estimate inherits the optimism and prices the indirects short.
The missing method statement. Without a written plan there is nothing to challenge, so the review argues about the total instead of the assumptions. The first version of the method gets written by the contractor, at tender, in their favour.
The forgotten window. Tidal windows, fish windows, traffic management hours and shutdown constraints surface at tender question period, after the budget is set. Each one is a productivity cut that was visible on day one to anyone who asked how the work would be built.
Why owners should insist on this
Because when a bid comes in 30 percent over, a method-based estimate can tell you why: the contractor assumed two spreads instead of one, or a shorter tidal window, or barge mobilisation from further away. The conversation becomes a comparison of plans, item by item, and it can be settled. A unit-rate estimate can only tell you that it was wrong, and the meeting that follows produces frustration instead of a decision.
The plan does not need to be elaborate. A page per major element, the spread, the productivity and the duration. Light enough to move, strong enough to hold.
Emerald Group builds method-based estimates for ports and marine, heavy civil and industrial projects, and reviews estimates and bids for owners who need to know which plan a number was built on. If your project is the kind where the method sets the cost, get in touch.