Prolongation costs, and the weeks they belong to
Weekly preliminaries multiplied by weeks of delay is the calculation everybody does first. It is almost always the wrong rate applied to the wrong period.
QScope Team · 21 May 2026 · 5 min read
Prolongation is the cost of keeping a site running for longer than planned. Site management, accommodation, welfare, temporary services, plant on standing hire, insurances: the costs that accrue with time rather than with output.
Where the delay is caused by a relevant matter, those costs are recoverable as loss and expense. Two questions decide the figure, and both are usually answered wrongly on the first attempt.
Which weeks
The instinctive answer is the weeks at the end: the job was due to finish in June and finished in September, so prolongation is three months of preliminaries in the summer.
That is generally not right. The costs are assessed over the period when the delay actually occurred, not over the period by which the end date moved.
If the delaying event happened in January and pushed everything back by twelve weeks, the additional site cost was incurred in January and the months following, when the site was manned as it was then. Those weeks may have carried a full site team, full accommodation and full plant. The weeks at the end may have carried a skeleton team finishing off.
Which rate
Not the tendered weekly rate. That is a price, and prolongation is a claim for loss.
The figure has to come from what was actually incurred: payroll for the site team, actual accommodation and welfare invoices, actual plant hire, actual insurance. If the contractor spent more than it allowed, the claim is for what it spent, provided it was reasonably incurred. If it spent less, the claim is for the lower figure.
Using the tender rate is attractive because the evidence is easy, and that is exactly why it is resisted. It proves what was priced, not what was lost.
What comes out
Costs already recovered elsewhere have to be excluded, or the same money is claimed twice. If time-related preliminaries have been valued in interim certificates over the extended period, that recovery reduces the claim.
This is where preliminaries valued as a percentage of measured work cause trouble. If nobody can say what has been certified against time-related preliminaries, nobody can say what is left to claim.
Concurrency
Where the employer’s delay runs concurrently with contractor delay over the same period, the general position is that prolongation is not recoverable for that period, even though an extension of time is granted.
The contractor would have incurred the cost anyway because of its own delay, so the employer’s event caused no additional loss. Time yes, money no.
Presenting it
Identify the event, the period of delay it caused, the site establishment during that period, the actual costs incurred, and the deduction for anything already recovered. Five components, each evidenced.
A claim in that form can be checked line by line. A weekly rate times a number of weeks can only be accepted or rejected, and it is usually rejected.
QScope keeps preliminaries as their own section and ties loss and expense to the delay event, so the period a claim covers is identifiable.