Most contracts value a variation by starting with the rates already in the contract, and most disputes about variation value start with an argument about whether those rates still apply. The test is not whether the rate is profitable but whether the work is of similar character executed in similar conditions, and that question is answered on the facts. Working through it is part of variation claim preparation.
Both sides get this wrong in predictable directions. Contractors argue that any difference justifies a new rate; employers argue that any similarity justifies the old one. The contract usually sets a more structured sequence than either.
The sequence most contracts set
First, where the varied work is of similar character and executed under similar conditions, the contract rates apply.
Second, where the character is similar but the conditions differ, the rates form the basis of the valuation with a fair adjustment for the difference.
Third, where the work is not of similar character at all, a fair valuation applies, built from first principles rather than from the bill.
Fourth, where none of those is appropriate, daywork may apply, usually subject to conditions about records and authorisation.
What similar character means
It refers to the nature of the work itself: the same operation, the same materials, the same trade, done in broadly the same way.
Laying a hundred square metres of the same screed is similar character to laying fifty. Laying a different screed, to a different specification, probably is not.
The test is applied to the work, not to the quantity. A substantial change in quantity may change the conditions without changing the character.
What similar conditions means
The circumstances in which the work is carried out: access, timing, sequence, working area, weather exposure, whether other trades are present.
This is where most disputes actually live, because the character is frequently identical and the conditions are plainly different.
Work instructed out of sequence, in an occupied area, at height where the original was at ground level, or in winter where the original was programmed for summer, is the same work in different conditions.
The contract response to that is usually not a new rate but the existing rate adjusted, which is a narrower argument and an easier one to evidence.
Rates that were wrong in the first place
A rate priced too low does not become inapplicable because the contractor regrets it. Where character and conditions are similar, the rate generally applies whether or not it is profitable.
The same holds in reverse. An employer cannot usually set aside a rate priced high on the basis that it now looks generous.
That symmetry is the point of using contract rates at all: both parties priced the risk at tender and both carry it. Front loading and unbalanced pricing are separate issues with their own consequences.
The evidence that decides it
For character: the specification of the original work and of the varied work, side by side, in enough detail to show whether they describe the same operation.
For conditions: records of how the work was actually carried out. Programme position, access arrangements, what else was happening on site, and what the original was priced to assume.
The last part is frequently missing. Where a contractor cannot show what conditions the rate assumed, it is harder to show that the actual conditions differed from them.
Tender build ups, where they exist, are therefore worth keeping. They are also worth preparing in a form that can be shown without disclosing more than necessary.
How this plays out in a claim
A variation claim that jumps straight to a new rate invites a response that the contract rates apply, and the argument restarts from the beginning.
A claim that works through the sequence, showing why each step does or does not fit, is harder to dismiss and frequently settles at step two.
That is usually the right answer in any event. Most varied work is similar in character and different in conditions, which points to adjusted rates rather than to fresh valuation.
Daywork is the last resort
Most contracts treat daywork as applicable only where the work cannot properly be valued by measurement, and frequently impose conditions about prior authorisation and contemporaneous records.
Those conditions are strict in practice. Daywork sheets signed late, or not signed at all, are the commonest reason daywork claims are reduced.
Treating daywork as the default for anything awkward is a reliable way to have a variation account refused, because it bypasses a sequence the contract set deliberately.
Quantity changes and their effect
A large increase or decrease in quantity can change the conditions even where the character is identical, because the original rate assumed a particular volume of work and the resources planned around it.
Some contracts address this expressly; others leave it to the general test of similar conditions. Either way the argument is about conditions rather than character.
The evidence needed is the same: what the original rate assumed, and what actually happened. Without the first, the second proves nothing.
Doing it as the project runs
Identify the valuation basis when the variation is instructed, not when the account is prepared. The information needed to show a change in conditions exists at the time and evaporates afterwards.
Agree the basis if you can, even where the figure is left open. An agreed rule removes most of the eventual argument.
That is ordinary cost control and variations work, and it is why projects with disciplined change control have small variation accounts rather than large ones. Where the account has already become contentious, the claims and dispute services deal with it as a defined exercise.
Variations that have not been agreed?
Variation claim preparation sets out the instructed scope, the measured difference and the valuation built on the rule the contract actually provides, with the evidence attached to each line.