What Are Prolongation Costs in Construction Contracts?
Prolongation costs are the additional time-related costs a contractor incurs when a project runs longer than planned for a reason that entitles the contractor to an extension of time — things like extended site establishment, extra supervision, additional plant hire and other preliminaries that keep running for every extra week the job stays on site. Where a delay is caused by the principal or another party outside the contractor’s control, and the contract grants an extension of time, prolongation costs are typically how the extra cost of that extended time gets recovered — separate from the value of any additional physical work.
Key takeaways
- Prolongation costs are the extra time-related costs of staying on site longer, not the cost of doing extra work.
- They typically arise from extended preliminaries — site establishment, supervision, plant hire, insurances — running over a longer period than planned.
- Entitlement to prolongation costs generally follows from an approved extension of time caused by something outside the contractor’s control.
- Recovering prolongation costs accurately depends on a well-itemised preliminaries build-up from the outset — it is hard to claim a rate that was never costed.
- This is general information about how prolongation costs commonly work, not legal advice — always check the specific contract wording.
What Prolongation Costs Actually Are
Prolongation costs cover the ongoing cost of keeping a job running for longer than the contract originally allowed, once an extension of time has pushed the completion date out. They are distinct from variation costs, which pay for additional scope — prolongation costs pay for extra time on the same scope, and they are usually claimed alongside, not instead of, an extension of time.
What Prolongation Costs Typically Include
Because prolongation costs are time-related, they are usually built from the preliminaries items in the original tender that were priced on a weekly or monthly basis rather than as a fixed lump sum.
- Extended site establishment — site sheds, amenities, fencing and hoardings
- Extended site supervision and management labour
- Extended hire of plant and equipment retained on site
- Extended insurances, bonds and other time-based charges
- A share of head office overheads attributable to the extended period
- Ongoing site utilities and services, such as power and water
How Prolongation Costs Relate to an Extension of Time Claim
An extension of time claim establishes the additional days or weeks a contractor is entitled to add to the contract period, usually because of a delay caused by the principal, a variation, or another event outside the contractor’s control. A prolongation cost claim is a separate step that quantifies the dollar cost of that additional time.
Not every extension of time carries an automatic right to prolongation costs — many standard-form contracts distinguish between an EOT that only protects the contractor from liquidated damages, and an EOT that also carries an entitlement to costs. Which applies depends on the specific clause and the cause of the delay, and delays caused by the contractor’s own default typically do not attract prolongation costs at all.
Time versus cost
Getting more time is not the same as getting paid for that time. Always check whether the relevant contract clause ties prolongation costs to the extension of time, or treats them as a separate entitlement that still needs to be proven.
Calculating and Documenting Prolongation Costs
A prolongation cost claim is only as strong as the preliminaries build-up behind it. Where preliminaries were priced as weekly or monthly rates in the original tender, that same rate can usually be applied to the extra period once an extension of time is agreed. Where preliminaries were lumped together as a single figure, working out a defensible weekly cost after the fact is far harder.
This is exactly why pricing preliminaries as itemised, time-based line items at tender stage matters — not just for prolongation claims, but for day-to-day accuracy. MTH’s editable rates library lets preliminaries be built up and priced by the week from the start, so if a delay does occur, the weekly cost of the extension is already sitting in the numbers rather than needing to be reconstructed under pressure.
| Preliminaries item | Weekly cost |
|---|---|
| Site supervisor | $2,400 |
| Site facilities and amenities | $350 |
| Plant and equipment retained on site | $900 |
| Insurances and bonds (proportioned) | $180 |
| Total illustrative weekly prolongation cost | $3,830 |
Frequently asked questions
What is the difference between prolongation costs and delay damages?
Prolongation costs are typically a contractor’s own claim for the extra cost of staying on site longer through no fault of their own. Delay damages, including liquidated damages, usually run the other way — a principal’s claim against a contractor for finishing late without a valid extension of time.
Do I automatically get paid prolongation costs if I get an extension of time?
Not automatically. Many contracts separate the right to extra time from the right to extra cost — whether prolongation costs are payable depends on the specific clause and on what caused the delay, so this should always be checked against the actual contract terms.
How do you calculate prolongation costs on a construction project?
Prolongation costs are usually calculated by applying the weekly or monthly rate for time-related preliminaries — supervision, site facilities, plant hire, insurances — to the additional period covered by the extension of time, then supporting that calculation with records showing the costs were actually incurred.
Can a subcontractor claim prolongation costs?
A subcontractor can potentially claim prolongation costs where its own subcontract entitles it to an extension of time and associated costs, and where the delay was not its own doing. The specific subcontract wording and the chain of causation back to the head contract both matter here.
What preliminaries items are usually included in a prolongation cost claim?
Common items include extended site supervision, site establishment and facilities, plant and equipment retained on site, insurances and bonds, and a proportion of head office overhead attributable to the extended contract period.
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