Two ways of asking for a price
A bill of quantities sets out measured quantities against described items, prepared to a measurement standard, so that every tenderer prices the same numbers. A schedule of works describes the work in words, room by room or element by element, and asks the contractor to price the description.
Both are legitimate and both are widely used. The choice is usually made on the basis of scheme size, which is a reasonable starting point and not the whole answer.
The better question is what you will need the document to do after the contract is signed, because that is where the difference shows.
A third option sits between them and is worth naming. A schedule of rates asks the contractor to price unit rates without committing to quantities, and it suits work where the extent is genuinely unknown at tender but the nature of it is well understood. It gives you rates for valuation while accepting that the total is provisional.
What each is good at
A bill gives you a genuine comparison between tenders, because everybody prices identical quantities. It gives you rates for valuing change. And it locates quantity risk with the party who prepared it, which is normally the employer.
A schedule is faster and cheaper to produce, suits work that is descriptive rather than dimensional, and handles refurbishment and fit out well where the scope is about what to do rather than how much there is. It transfers quantity risk to the contractor, who has to satisfy themselves about extent.
On smaller works and on repetitive alteration work a schedule is often the right tool. On anything where change is likely and value is significant, the bill earns its cost the first time something has to be revalued.
Size is a reasonable proxy and it is not the test. A small fit out with a demanding client and a moving brief will generate more change than a larger new build with a settled design, and the smaller job is the one that needs rates. Ask how many variations you expect rather than how many square metres there are.
Valuing change is where they diverge
When a variation arrives, the contract usually values it from rates already in the pricing document, then from rates analogous to them, then by fair valuation where nothing comparable exists.
A bill puts you at the top of that hierarchy for most changes, because there is a rate for a similar item. A schedule with lump sums against descriptions frequently puts you at the bottom, where the valuation is negotiated from first principles with the contractor holding the information.
That is the practical cost of the cheaper document, and it arrives later, which is why it is easy to discount at the point of choosing.
The hybrid that usually works
On a lot of schemes the sensible answer is neither pure form. A schedule of works for the descriptive elements, with a measured bill for the parts that carry most of the value and most of the change risk: substructure, frame, envelope, and any element with high rates and uncertain extent.
That keeps the document proportionate while retaining rates where they will be needed. It requires somebody to decide which parts belong in which, which is a commercial judgement rather than a measurement one.
Whichever you choose, the descriptions carry it
The most common failure in both documents is thin description. An item described without its context, its access, its sequence or its finish standard will be priced against the easiest reading of it.
On refurbishment particularly, the description is doing most of the work. We covered the underlying discipline in NRM2 measurement explained and the purpose of the bill in bill of quantities purpose.
What this means for you
Choose the pricing document by asking how much change you expect and how much value sits in each element, not only by asking how big the job is.
Where the decision is finely balanced, the cost of preparing a bill is knowable now and the cost of not having one is not. That trade is exactly what quantities and take-off work is for, and it is described on our client side quantity surveyor page.
Whatever the document, insist that preliminaries are broken into time related and fixed elements. That single requirement costs a tenderer almost nothing and it is what allows any later change in duration to be valued sensibly. Without it, a programme extension becomes an argument about a lump sum that was never explained.