Prolongation Cost Claims: How to Calculate and Claim
You have your Extension of Time. The project ran long, your team stayed on Site, the cabins stayed rented, and now you want the money back. This guide shows how to calculate a prolongation cost claim properly — the heads of cost, the right period, the records, and how to present it so the Engineer pays rather than picks it apart.
What Prolongation Cost Is — and What It Is Not
Prolongation cost is the additional time-related cost the Contractor incurs because the Works took longer than they should have, through no fault of its own. Think site overheads, staff, plant standing by, and accommodation that kept running while the job dragged on.
It is the money side of delay. An Extension of Time protects the Contractor from liquidated damages by moving the completion date. Prolongation cost is the separate claim for what that extra time actually cost to be on Site.
What it is not: it is not the same thing as your tender preliminaries, and it is not a reward for finishing late. It is reimbursement of real, evidenced cost caused by an event that carries a money entitlement under the contract.
A useful test is the "but for" question. But for the Employer's delay event, would the Contractor have incurred this cost? If the answer is no, it is likely prolongation. If the answer is "we would have spent it anyway," it is not — and trying to claim it anyway is how good claims get a bad reputation.
Key Takeaway: Prolongation cost is the extra time-related cost of being on Site longer because of an Employer delay event. It is the money claim that sits next to an Extension of Time — not a daily rate, and not a finishing-late bonus.
Prolongation vs Disruption vs Acceleration
These three get mixed up constantly, and mixing them up is the fastest way to have a claim rejected on principle. They describe different things, prove different facts, and are measured in different ways.
- Prolongation — the Works took longer. You claim the time-related cost of the extended duration on Site (overheads, staff, standing plant).
- Disruption — the Works became less efficient. You claim the lost productivity, even if the end date did not move. A crew that should lay 100 metres a day managing only 60 is disruption.
- Acceleration — you sped up to recover or beat a delay. You claim the cost of the extra resources, shifts, or overtime used to do it.
The clean way to remember it: prolongation is about being there longer, disruption is about working slower, and acceleration is about working harder. A single delay event can trigger all three, but each needs its own analysis and its own evidence.
Bundling them into one round number labelled "delay and disruption costs" is common, and it is also the claim the Engineer enjoys cutting the most. Keep them separate or watch them get treated as one suspicious lump.
Key Takeaway: Prolongation is being on Site longer, disruption is working less efficiently, acceleration is working faster to catch up. They prove different facts and are measured differently — never merge them into one number.
What You Can Actually Claim For (Heads of Cost)
A prolongation claim is built head by head. Each head is a category of time-related cost that genuinely increased because of the extended duration. The usual heads are:
- Site overheads / preliminaries — the running cost of keeping the Site open: site management, security, welfare, temporary services, small tools, consumables.
- Site staff — the Project Manager, Site Supervisor, engineers, foremen, and Quantity Surveyor retained on Site for longer than planned.
- Plant and equipment — owned or hired plant standing by or retained on Site through the delay period. Hire invoices are gold here.
- Accommodation and facilities — site cabins, offices, camps, and the cost of keeping them through the extended period.
- Head-office overhead — the share of central running costs (directors, accounts, IT, head office) carried by the contract while its resources stayed tied up. Claimed on actuals where possible, or by formula.
- Finance / interest — the cost of money tied up for longer, where the contract and the law allow it.
Notice what is not on the list: profit on the prolonged period is rarely recoverable as a head of cost, and the loss of a future contract you say you could have won is almost impossible to prove. The heads that win are the boring ones with invoices attached.
Key Takeaway: Build the claim head by head: site overheads, site staff, plant, accommodation, head-office overhead, and finance. The recoverable heads are the ones backed by invoices and timesheets — not lost profit or hypothetical lost work.
How to Calculate Prolongation Cost
Here is the single most important rule, and the one most claims get wrong. Prolongation cost is not your preliminaries bill rate multiplied by the days of Extension of Time. That gives a tidy number and an unpersuasive claim.
The Society of Construction Law Delay and Disruption Protocol is clear on the principle: prolongation cost is assessed on the cost actually incurred during the period the delay event had effect — not a daily rate taken from the bill of quantities, and not the cost at the very end of the job.
The method, in order:
- Fix the period of effect. Identify when the delay event actually delayed the Works on the critical path. This is the assessment window — more on it in the next section.
- Pull the actual cost in that window. For each head, gather what was really spent during that period: timesheets, hire invoices, accommodation costs, allocation sheets.
- Strip out what is not caused by the delay. Remove cost the Contractor would have carried anyway, and any cost already recovered elsewhere or caused by its own default.
- Assess head-office overhead. Use actual records if you have them; use a recognised formula if you do not.
- Add finance, if allowed, and total it up by head with the evidence cross-referenced.
Why the actual-cost route and not the bill rate? Because the preliminaries rate was a price, set at tender to win the job and value the work. It was never a measurement of what the delay cost. The Engineer knows this, and a claim built on the rate invites the reply that the rate proves nothing. Tools like ChatNotice help structure the underlying notice and particulars so the cost story lines up with the contractual entitlement from the start.
Key Takeaway: Calculate prolongation on the actual cost incurred during the period the delay had effect, not on the preliminaries daily rate from the bill. The SCL Protocol assesses cost in the period of delay — the rate is a price, not proof.
The Right Period: Why the Window Matters
This is the point that separates a defensible claim from a guess. The relevant period for assessing prolongation cost is the period when the delay event had its effect — not the overrun tacked on at the end of the job.
It feels natural to say: the job finished three months late, so I will claim three months of overheads from the tail end of the programme. It feels natural, and it is usually wrong. The cost in the final three months may be much lower or much higher than the cost during the period the event actually bit.
Picture a delay event that hit early, when the Site was at peak resource — full crew, all the plant, the big cabins. The real cost of that delay sits in those expensive early weeks, not in the quiet demobilisation period at the end when half the team has already gone home.
The SCL Protocol approach is to value prolongation in the period the cause had effect. So the analysis links the delay event to a specific window on the programme, and the cost is drawn from that window. Claim the wrong months and you can be over or under by a wide margin — and either way you have handed the Engineer a reason to reject the basis.
Key Takeaway: Value the cost in the period the delay event had effect, not the overrun at the end. The end of a job is often the cheapest part to run, so claiming the tail months can quietly understate — or overstate — the real loss.
Records That Make or Break the Claim
A prolongation claim lives or dies on its records. Entitlement might be sound, the period might be right, and the claim still fails if you cannot show the money actually went out the door during that window.
The records that carry the weight:
- The accepted baseline programme and the as-built record — to fix the period of effect and link cost to it.
- Dated daily reports — labour, plant, and activity on Site, day by day.
- Allocation sheets and timesheets — who and what was on Site, and for how long.
- Hire invoices and plant returns — proof that plant was genuinely standing or retained.
- Accommodation, facility, and supplier invoices — the running cost of the Site through the period.
- Payroll and staff cost records — for the retained site team.
- Company accounts — if head-office overhead is claimed on an actual or Emden basis.
The hard truth is that the best time to win a prolongation claim is months before you write it — by keeping clean, dated, contemporaneous records as the job runs. Records reconstructed after the fact have a way of looking exactly like what they are. The Quantity Surveyor who keeps the daily diary up to date is doing claim-winning work, even if nobody thanks them for it at the time.
Key Takeaway: The claim stands on contemporaneous records: baseline and as-built programme, daily reports, timesheets, hire invoices, accommodation costs, payroll, and accounts. Records kept during the job beat anything reconstructed after it.
Concurrent Delay and Its Effect on Cost
Concurrent delay is when two causes of delay run on the critical path at the same time — typically one the Employer is responsible for, and one the Contractor is responsible for. It is the most argued-about topic in delay, and it bites cost harder than it bites time.
The key point for a prolongation claim: time and money are treated differently. Under the Malmaison approach commonly applied in English-law contracts, true concurrency may still give the Contractor the Extension of Time, yet give no prolongation cost for the overlapping period.
The logic is straightforward once you see it. If the Contractor's own delay would have kept the Site open for that period anyway, the Employer's concurrent delay caused no additional cost. No additional cost means nothing to reimburse, even though the time is still extended to protect against liquidated damages.
So you can win the time and lose the money for the same weeks. That outcome surprises people every time, usually right after they have already spent the money in their heads. Other jurisdictions apportion or take a different view, so the contract and the governing law decide the exact treatment — but the warning stands.
Key Takeaway: Concurrency hits money harder than time. Under the Malmaison approach you can keep the Extension of Time yet recover no prolongation cost for the overlapping period, because your own delay would have carried that cost anyway. The contract and jurisdiction set the rule.
How to Present a Prolongation Claim
A persuasive prolongation claim tells one clear story: here is the entitlement, here is the period it affected, here is the cost in that period, and here is the evidence for every figure. Each step should hand the Engineer the proof for the next.
The structure that works:
- Entitlement — the cause, the clause that carries cost, and the Extension of Time already granted or claimed for the same event.
- The period of effect — the programme analysis linking the event to a specific window, with the baseline and as-built shown.
- Cost by head — each head set out separately, valued on actual cost in that window, cross-referenced to the records.
- Head-office overhead — the basis stated plainly (actual, Hudson, Emden, or Eichleay) and the inputs shown.
- Concurrency treatment — addressed honestly, with any reduction explained.
- Evidence bundle — the invoices, timesheets, and reports, indexed so each figure can be traced.
Tie the cost claim back to the same delay event and the same Extension of Time. A prolongation claim that floats free of the time claim invites the question of what it is actually for. Keep the narrative tight, let the records carry the numbers, and resist the urge to round up — a precise, evidenced figure earns more trust than a confident large one.
Key Takeaway: Present entitlement, period of effect, cost by head, overhead basis, concurrency, and an indexed evidence bundle — all tied to the same delay event and Extension of Time. Let the records carry the numbers, and do not round up.
Frequently Asked Questions
Is prolongation cost the same as my contract preliminaries daily rate?
No. The preliminaries daily rate in the bill is a tender figure used to price and value work — it is not proof of what the delay actually cost. Prolongation cost is assessed on the actual time-related cost the Contractor incurred during the period the delay event had effect. The two numbers may differ a lot, and the Engineer is entitled to ask for the actual cost, not the rate.
Can I claim head-office overhead, and how do the Hudson, Emden and Eichleay formulae work?
Often yes, but it is the hardest head to recover. Where actual records exist, claim the real, evidenced under-recovery. Where they do not, formulae estimate it: Hudson uses the tendered overhead and profit percentage, Emden uses the company's actual overhead percentage from its accounts, and Eichleay allocates head-office overhead to the contract by billing proportion. Each requires you to show the resources were genuinely tied up and could not earn elsewhere.
Do I get prolongation cost for a weather delay?
Usually not. Under the FIDIC 1999 Red Book, exceptionally adverse weather under Clause 8.4(c) gives an Extension of Time only, with no entitlement to cost. So the project can be delayed, the time can be extended, and yet there is no prolongation money for that period. Cost follows the specific cause clause — Variations and Unforeseen Physical Conditions give cost; weather generally does not.
What records do I need to support a prolongation cost claim?
The accepted baseline programme and the as-built record, dated daily reports, allocation sheets for staff and plant, hire invoices, accommodation and facility costs, payroll for retained site staff, and the company accounts if you are claiming head-office overhead. The records must cover the period the delay had effect, not just the overrun at the end. Cost claimed without contemporaneous records is the first thing the Engineer cuts.
How does concurrent delay reduce my prolongation cost claim?
Time and money are treated differently. Under the common Malmaison approach in English-law contracts, true concurrency may still give the Contractor the Extension of Time, but it does not give the prolongation cost, because the Contractor would have incurred that cost anyway due to its own concurrent delay. So you can win the time and still lose the money for the overlapping period. The contract and jurisdiction decide the exact treatment.