A defined provisional sum is one where the bill states the nature and construction of the work, how and where it fits into the building, its scope and quantity, and any limitations on how it is carried out. An undefined provisional sum states none of that.
The distinction matters because a contractor is deemed to have allowed in their programme and their preliminaries for defined work, and is not deemed to have allowed for undefined work. That single difference decides who carries the time and the on cost when the sum is instructed.
What defined actually requires
Not an approximate description. The measurement rules require specific information: what the work is, how it is constructed, where it sits in the building, its scope, its quantity, and any constraints on sequence, access or method.
The test is whether a contractor reading it could plan and price around it. A sum described as allowance for drainage works fails that test. A sum describing a stated length of a stated size of drainage in a stated location, to be carried out during a stated period, passes it.
Meeting the requirement takes design information. Where that information does not exist, the sum cannot honestly be described as defined, and calling it defined does not make it so.
The commercial consequence
Where a defined sum is instructed and the work takes longer or requires more resource than the contractor allowed, that is their risk. They had the information and were deemed to have planned for it.
Where an undefined sum is instructed, the contractor is entitled to the time and the preliminaries consequence, because the contract accepted that they could not have planned for it.
On a project with several large undefined sums, the client is exposed to a series of programme and preliminaries claims that will arrive one at a time as each sum is instructed, and each will be individually reasonable.
Why undefined sums proliferate
Time pressure, mostly. A scheme goes to tender before the design is complete, and the parts that are not resolved become provisional sums because there is no alternative.
That is a legitimate response to a real constraint. The failure is not using them; it is using them without recognising that each one is a piece of the price that has not been competed and a piece of the programme that has not been fixed.
A tender where a large proportion of value sits in provisional sums is not a fixed price. It is an estimate with a contract attached, and it should be reported to the client in those terms.
Reducing exposure without the design
Where the design genuinely cannot be completed, several things still help. Describe as much as is known, so the sum moves closer to defined even if it does not reach it. State the assumed period and sequence, which limits the programme argument.
Fix the rates that will apply if the work turns out to be similar in character to measured work elsewhere, which removes the valuation argument. Set a date by which the information will be issued, which makes the constraint visible.
Each of those converts part of an open risk into a defined one. None requires the design to be finished.
Refurbishment is the hard case
On refurbishment the information may not exist at any price until the fabric is opened up, so provisional sums are unavoidable and often numerous.
The answer there is the opening up protocol: agreed before contract, stating who opens up, when, in whose presence, what record is made and how the finding is valued. That converts a series of arguments into a series of administrative steps.
It also means the survey programme matters as much as the design programme, which is set out further under refurbishment and reuse.
Expenditure and the account
When a provisional sum is instructed, the sum in the contract is omitted and the actual value of the work is added. Both figures should appear in the account so the reader can see allowance against expenditure item by item.
Where the expenditure exceeds the allowance repeatedly, that is information about how the scheme was documented rather than about how it was built, and it is worth capturing for the next project.
The mechanics of that adjustment sit in how a final account is actually assembled.
What this means for you
Ask which of your provisional sums are defined and which are not, and ask for the total value of each category. That single number tells you how much of your fixed price is actually fixed.
Then ask what information would move the undefined ones into the defined category and what it would cost to get it. On most schemes it is cheaper than the exposure it removes. The related failure is described in quantity assumption risk.
Have a bill full of provisional sums?
Send it with the drawings. We will tell you which are defined, which are not, and what that means for your exposure.