Extension of time and loss and expense: how to calculate what you are owed
In short
An extension of time protects you from liquidated damages but does not pay your additional costs. To recover money, you need a separate loss and expense claim built on actual prolongation and disruption costs, each tied to dated site records.
- An extension of time removes your exposure to liquidated damages but does not recover your costs; loss and expense is a separate financial claim with its own qualifying conditions.
- Under JCT contracts, relevant events give time while relevant matters give money - the two lists overlap but are not identical, so always check both before assuming entitlement.
- Prolongation costs must be based on actual expenditure incurred during the specific delay period, not a weekly rate averaged from the bill of quantities or the tail-end overrun period.
- Disruption is a separate head of claim covering lost productivity on activities that did not extend the programme - it requires measured mile or benchmarked evidence, not a programme analysis.
- Dated records - site diaries, timesheets, invoices, allocation sheets - are the difference between a claim that settles and one that is rejected on evidential grounds.
An extension of time removes your exposure to liquidated damages but does not recover your costs. This guide explains how to calculate prolongation, disruption, and extended preliminaries - and what dated records you need to substantiate each head of claim.
An extension of time and a loss and expense claim are two separate things, and only one of them puts money in your account. An extension of time (EOT) shifts the completion date forward so that the employer cannot deduct liquidated damages for delayed handover. It is a shield against penalty, not a payment mechanism. The money comes from a separate loss and expense claim, which must be built on a different set of qualifying events and supported by dated financial records. Many contractors on construction projects win the time and lose the money because they treat the two as the same process.
Why an EOT Does Not Automatically Pay You
Under JCT contracts, two parallel lists govern entitlement. "Relevant events" entitle the contractor to an extension of time. "Relevant matters" entitle the contractor to loss and expense. The lists overlap but are not identical. Some relevant events - adverse weather is the clearest example - give time but carry no right to money. The contractor gets relief from liquidated damages but cannot recover the additional weeks of site overheads, supervision, or plant costs that ran during the extended period.
Under NEC4, the mechanism is different: "compensation events" provide entitlement to both time and money in a single assessment. But the need to notify within the contract's time bar, and to calculate the financial impact separately from the programme impact, is the same discipline regardless of which form you are working under.
The practical consequence is this: if you have been granted an eight-week EOT but have not filed a loss and expense application, you have protected yourself from a damages deduction - and that is all. The cost of eight additional weeks of site management, preliminaries, and disruption to your workforce remains unrecovered.
Understanding the Three Heads of a Loss and Expense Claim
A loss and expense claim usually contains three separate components, each calculated differently.
Prolongation costs are the time-related expenses you incur because the overall project duration has been extended. These are preliminaries items: site management salaries, site accommodation rental, welfare facilities, temporary power, hired plant, and security. If your project was planned to take 40 weeks and now takes 48, you have incurred eight additional weeks of these costs. They are distinct from the direct cost of carrying out the works themselves.
Disruption costs are different. Disruption relates to events that reduce your productivity without necessarily extending the overall programme. You may have been forced to resequence activities, work around other trades you were not expecting, break out and redo work, or wait for late information while keeping a workforce on site. Disruption costs appear as lost efficiency: labour that was on site but could not work productively, or the same output achieved at a higher unit cost because of interference. RICS guidance notes that attributing loss of productivity to a specific relevant matter is more challenging than quantifying prolongation, which is why site records become critical.
Head office overhead is the third component. Contractors allocate a proportion of their central overhead - finance, HR, management - to each project as a percentage of turnover. If a project runs over, a larger portion of that overhead is consumed. Courts have accepted formula-based approaches such as the Emden and Hudson formulae to calculate this element where direct allocation is not possible, though actual cost evidence is preferred where it can be produced.
The Critical Rule on the Period of Assessment
One of the most common errors in prolongation claims is measuring the wrong time period. The correct approach is to calculate the costs incurred during the period when the delaying event actually occurred - not the period at the tail end of the project when the extra time was used up.
If a 12-month project experiences a two-month employer delay in month six, the recoverable costs are those incurred in month six and seven, not months thirteen and fourteen. This matters because preliminaries costs are not uniform across a project. They tend to be highest during the mobilization and heavy construction phases, and lower toward the end. Using the extended period at the tail of the project to calculate a weekly rate will produce a figure that does not reflect actual loss.
The corollary is that you cannot simply take the total preliminaries in the bill of quantities, divide by the contract period, and multiply by the delay weeks. That produces a theoretical average, not an actual cost. Courts have consistently rejected theoretical rates in favour of demonstrated actual expenditure during the specific delay period.
How to Calculate What You Are Owed
Confirm the qualifying event and check both lists
Identify the delay event and check whether it appears on the relevant matters list (JCT) or qualifies as a compensation event (NEC). Do not assume that because you have an EOT entitlement you also have a loss and expense entitlement - check the specific contract provision. Note the date the event started affecting the works and the date it was resolved or absorbed.
Identify the precise delay period
From your programme analysis, establish exactly which weeks were affected by the qualifying event. Use a contemporaneous programme that was updated regularly during the project, not one reconstructed after the fact. The delay period for cost calculation is the window during which the delaying event was in operation on the critical path, not the project overrun at practical completion.
Extract actual prolongation costs for that period
Pull together all time-related costs incurred during the identified delay period. Include site management salaries with payroll records, accommodation invoices, hired plant invoices, welfare and security costs. Do not use the BoQ rate or a weekly average - extract the actual expenditure for the specific weeks in question. Where plant or accommodation is shared across two projects, allocate proportionally and document your basis for doing so.
Calculate disruption costs separately
Disruption requires a different evidential approach because you are quantifying lost productivity rather than additional time on site. Methods include the measured mile - comparing productivity on affected activities against productivity on the same type of work earlier in the project when there was no disruption - or a comparison against industry benchmarks. Gather your labour allocation sheets, daily output records, and any instructions or drawings that caused resequencing. Mark each record with the date received and the activity affected.
Quantify extended preliminaries line by line
List each preliminary item individually. For site management and supervision, use salary payslips or subcontractor day-rate invoices. For hired plant and equipment, use hire invoices. For temporary accommodation, use rental invoices. The aim is a line-by-line schedule that traces each cost to a dated document. Estimates and approximations will be challenged; actual invoices will not.
Add head office overhead if applicable
If your contract sum includes a head office overhead percentage, calculate the amount absorbed by the delay period. If you can trace actual additional head office costs - additional project management time, legal or QS fees incurred in managing the delay - claim those directly with supporting timesheets and fee notes. The Hudson or Emden formula may be used to calculate the overhead contribution, but present it alongside any actual-cost evidence you hold.
Assemble and date-stamp your evidence
Every figure in the claim needs a corresponding document dated at the time the cost was incurred. Organize your evidence by head of claim and by week. Site diaries recording daily weather, workforce numbers, activities underway, and instructions received are foundational. Timesheets, invoices, photographs with timestamps, allocation sheets, and programme updates all serve to establish the causal link between the qualifying event and the cost claimed. A claim that totals the right number but cannot point to dated records for each line is vulnerable to a wholesale rejection.
Issue notice under the contract and track the application
Most standard form contracts make formal notice a condition precedent to recovery. Under JCT, the contractor must make a written application to the contract administrator as soon as it becomes apparent that regular progress has been materially affected. Under NEC, early warning notices and compensation event notifications carry strict time bars. Issue notices at the point the delay event arises, not at the point you begin quantifying the claim. Use your project tracking system to log notice dates, response deadlines, and the current status of each application so nothing falls through the gap between site and commercial teams. The job record for each contract is the right place to attach programme updates, delay notices, and correspondence as they are issued, creating an audit trail that runs alongside the claim.
Present the claim as a schedule, not a narrative
The final claim document should lead with a schedule: one row per head of claim, showing the qualifying event, the delay period, the calculation method, and the total. Supporting evidence is annexed by reference. A narrative explaining what happened is useful context, but it is the schedule that the contract administrator or QS will mark up and return. A well-organized schedule that separates prolongation, disruption, and head office overhead - and shows the evidential basis for each - moves faster through the assessment process than a narrative document that buries the numbers in prose.
What Gets Claims Rejected
Prolongation claims fail most often for one of three reasons. The first is using theoretical costs rather than actual costs - an average weekly rate calculated from the bill of quantities rather than real invoices and payroll records. The second is claiming for the wrong period - measuring the tail end of the project rather than the delay window. The third is inadequate notice: where the contract makes notice a condition precedent, a claim submitted without the required notice may be rejected entirely on procedural grounds, regardless of its underlying merit.
Concurrent delay is a fourth risk. If the contractor was also in delay during the same period for its own reasons, the employer may argue that the prolongation costs would have been incurred regardless, and the claim is reduced or extinguished even where the EOT is preserved.
Notice is not optional
Under most standard forms, failing to issue a timely written notice of delay can bar recovery entirely, even where the entitlement is clear and the costs are well documented. Issue notice at the point the event arises, not when you start writing the claim.
Record the delay period, not just the extension
Your site diary and programme updates should record what was happening during the specific weeks the delaying event was in operation. A retrospective narrative is far harder to defend than a daily site diary maintained throughout the project.
The difference between an extension of time that costs you nothing and a loss and expense settlement that recovers your actual overrun costs is almost always a records problem, not a legal one. Contractors who maintain dated, itemized records throughout a project - logging delay events as they happen, attaching supporting documents to the job record in real time - are in a position to quantify what they are owed. Those who reconstruct the claim from memory or estimate the numbers after practical completion face a much harder argument.
Sources
- Prolongation costs in construction disputesPinsent Masons (Out-Law) · accessed 2026-10-14
- Prolongation vs. Disruption Claims - Easy QS GuideMetroun · accessed 2026-10-14
- Claims for Preliminary Costs - What is Included?Claims Class · accessed 2026-10-14
- Loss and expense: JCT claims process and examplesQredible · accessed 2026-10-14
- Extensions of time and liquidated damagesPinsent Masons (Out-Law) · accessed 2026-10-14