A bill of quantities is produced from the drawings and the specification, usually by a quantity surveyor. It breaks the works into items, states a unit of measurement for each, and states how much of it the job needs. Excavation in cubic metres. Blockwork in square metres. Reinforcement in kilogrammes. Doors by number.
What arrives is a document of many pages that most owners open once and close again. That is a mistake, because the BQ is the only thing on a build that lets you answer the question every owner eventually asks, which is whether the money going out is buying the work that was supposed to be bought.
The two jobs it does
Job one: getting comparable prices. Because every contractor prices the same quantities, the difference between two tenders is a difference in rates, not a difference in what each one assumed. Without a BQ you are not comparing prices. You are comparing guesses about scope, and the cheapest guess usually turns out to be the one that left the most out.
Job two: knowing where you stand mid-build. This is the job that gets abandoned, and it is the more valuable of the two. A BQ says the job needs 420 bags of cement. When 500 have been delivered and the blockwork is two-thirds up, that is a question worth asking now, while the answer is still recoverable. A build with no measured baseline cannot ask it. "We have spent four million" means nothing until you know what four million was supposed to buy.
How the document is arranged
Most bills follow the same order, and knowing the order makes the document navigable.
Preliminaries come first: site set-up, temporary works, supervision, insurance, water and power on site. Nothing in this section is visible in the finished house, which is why owners skim it. It is frequently a large share of the contract sum, and it is where a thin quote hides its optimism. See preliminaries.
Measured works follow, arranged by element or by trade: substructure, frame, walling, roof, finishes, fittings, services, external works. Each item has a description, a unit, a quantity, a rate and an amount.
Provisional sums and prime cost sums sit wherever the scope is not yet settled. These are allowances, not prices, and they are expected to move. See provisional sum.
Contingency, where the bill carries one, is a stated allowance for the unforeseen rather than for the unfinished.
A worked example
Suppose the walling element is measured at 380 square metres of 200mm blockwork, and a contractor prices it at KES 2,400 per square metre. The element is KES 912,000.
Four months in, three of four elevations are up to lintel level. Call that roughly three-quarters complete, and roughly is enough. Three-quarters of KES 912,000 is KES 684,000.
Now add up what has actually gone into that element: blocks counted in on goods received notes, sand and cement issued to it, the mason's labour posted against it. If the answer is KES 690,000, the element is on track. If the answer is KES 810,000, you have found a problem in month four rather than at handover, and a problem found in month four is still a decision rather than a bill.
That comparison takes about ten minutes a week for the two or three elements in progress. It is the entire discipline, and almost nobody does it.
What goes wrong
The quantities stop matching the building. Drawings change and the bill does not get remeasured, so the baseline quietly stops describing the thing being built. A BQ that has not been updated for four months of variations is a historical document.
Rates get treated as fixed. They are a bidder's view of the market on the day they priced. Cement moves, steel moves, and a rate agreed nine months ago is not a promise about today.
Nobody uses it after tender. This is the commonest failure by a wide margin. The bill did its job on tender day and then went into a drawer, which means the owner paid a quantity surveyor for half of what the document was for.
What it is not
A BQ is not a budget and it is not a contract sum. It is a measurement. Rates are applied to it to produce a price, and both the quantities and the rates can be wrong: quantities because the drawings changed, rates because the market moved. Treat it as the thing you measure reality against, not as a promise about what reality will cost.