What a bill of quantities actually covers
Almost every dispute we have been asked to look at began as a misunderstanding about one document.
The pattern is always the same. The client believed a number was fixed. The contractor believed it was measured against a drawing. Both were reading the same document, and only one of them had been told what it was.
What it is
A bill of quantities lists every measured item of work on a project — cubic metres of concrete, square metres of blockwork, linear metres of pipe, number of doors — with a rate against each. Multiply, total, and you have a price.
The quantities are measured from the drawings, not estimated. That is the whole point: two contractors pricing the same bill are pricing the same building, so the difference between their numbers is genuinely the difference between the contractors, not between what each of them imagined.
What it does not cover
It does not cover what is not drawn. If the drawings show no soakaway, no line in the bill pays for one. The day you decide you want it, the price changes — and that is arithmetic, not opportunism.
It does not cover your changes of mind. Moving a wall after the blockwork is up means paying to build it, paying to take it down, and paying to build it again. The bill priced the first wall.
And it does not cover what nobody could see. Rock two metres down, a water table higher than the survey suggested, a neighbour's foundation encroaching on your line. Good contracts say in advance who carries that risk. Bad contracts leave it to be argued about in month four.
How to read one without being a quantity surveyor
Three questions get you most of the way.
Is there a provisional sum, and what is it hiding? A provisional sum is a placeholder for work not yet designed. One or two is normal. A bill thick with them is a price that has not been worked out yet.
Are the preliminaries priced or buried? Preliminaries are site huts, hoarding, security, insurance, supervision — the real costs of running a site. A contractor who shows them is telling you what they cost. One who spreads them invisibly into the rates is hoping you compare only the headline.
What is the rate for the thing you are most likely to add? More tiling, another socket, extra paving. Ask now, while you have competing prices, not later when you have one contractor and no leverage.
The honest summary
A bill of quantities is not a promise that a building will cost this. It is a promise that this scope costs this. Every argument about construction cost is really an argument about scope, and the bill is where scope is written down.