A bill of quantities is a schedule describing and quantifying every item of work in a project, prepared to a measurement standard so that different contractors price the same thing.
It is frequently described as an outdated document, largely by people whose projects have not yet needed one. It does four separate jobs, and the arguments about whether it is worth preparing usually consider only the first.
Job one, a comparable basis for pricing
Every bidder prices the same quantities, so the differences between bids reflect rates and method rather than measurement. That is the only way a tender comparison means anything.
Where each contractor measures their own work, the totals differ for reasons nobody can see, and the client is choosing between four different scopes presented as four prices for one.
This is the job everybody knows about, and on design and build it is deliberately transferred to the contractor along with the risk. That transfer is a legitimate choice with consequences, covered in JCT Design and Build, where the risk really sits.
Job two, a mechanism for valuing change
This is the job that gets forgotten and it is worth more than the first on any project with change.
A priced bill provides rates for work of similar character executed under similar conditions. When a variation arrives, there is an agreed rate to apply, and the argument is about whether the rule applies rather than about the price.
Without one, every variation is valued from first principles or negotiated, at a point when the contractor already holds the contract and there is no competitive tension. Over a project with substantial change that difference is significant, and the hierarchy is set out in valuing a variation under the contract rules.
Job three, a basis for interim valuation
Monthly valuations against a priced bill are a measurement exercise with a defined answer. Against a lump sum with no breakdown, they are a negotiation about percentage completion.
That difference shows up every month for the life of the project, and it is where a great deal of low grade disagreement comes from.
It also affects the client's ability to know what has actually been paid for, which becomes acute if a contractor fails part way through.
Job four, a description of the works
A bill describes what is being built in a form that can be checked against the drawings. Preparing one therefore audits the design, and the process routinely finds gaps, inconsistencies and elements nobody had resolved.
Those findings arrive before tender, when correcting them is free, rather than during construction when it is not.
This benefit accrues even where the bill is never used for its other purposes, and it is why the exercise is worth something on a design and build scheme too.
When it is not worth it
On a small, simple project with little expected change and a complete design, a schedule of works or a specification and drawings may be sufficient and considerably cheaper.
On a scheme where the design is genuinely incomplete, a bill will be full of provisional sums and approximate quantities, and it will look precise while being nothing of the sort.
The honest test is whether the design supports measurement. Where it does not, other documents serve better, and the difference is covered in the discussion of schedules of work.
The residential case
Repeated unit types make measurement efficient and make the rates highly reusable across the scheme, which strengthens the change valuation argument.
They also make errors expensive, because an error in one unit type multiplies across every instance. That cuts both ways and it is an argument for the measurement being checked rather than for skipping it.
The residential context is set out under residential development.
What this means for you
Decide on the basis of how much change you expect, not on the basis of whether bills are fashionable. On a scheme with a complete design and little anticipated change, the case is weaker. Equally, on anything else it is strong.
And if you are not preparing one, make sure the tender documents still require a priced breakdown detailed enough to value change and interim payments against. That is the minimum, and it is frequently missing, as described in quantity assumption risk.
Deciding whether your scheme needs a bill?
Tell us the procurement route and the design stage. We will set out what a bill would buy you and what it would cost.