An Extension of Time is a change to the contract's completion date. Its effect is relief: for the extended period the Contractor is not late, so liquidated damages do not apply. It does not, on its own, put money in the Contractor's hands. Prolongation cost is the separate claim for the Contractor's own time-related costs of being on site longer than planned, and it must be proved on its own terms.
Side by side
| Extension of Time | Prolongation cost | |
|---|---|---|
| What it is | Additional contract duration | Compensation for time-related cost during the extended period |
| What it gives | Relief from LDs for the extended days | Money |
| Basis | The event is an Employer or neutral risk that delayed completion | The event is one the contract makes compensable, and it caused cost |
| Proof | Cause, effect on the critical path, duration | The cost actually incurred, tied to the delay period, with records |
| Neutral events | Often give time (for example exceptional weather under many forms) | Usually give no money |
| Concurrency | May still allow time, depending on the approach taken | Typically defeats or reduces the money claim |
| Where argued in the claim | Time impact section | Separate cost section or separate claim |
Why neutral events split the two
Many standard forms grant an extension for events that are nobody's fault, such as exceptionally adverse weather or force majeure, because it would be unfair to charge the Contractor LDs for them. The same forms do not usually compensate the Contractor for those events, because it would be equally unfair to charge the Employer. So an EOT for weather relieves LDs and yields no prolongation cost, while an EOT for late drawings may do both. The register should record, for each event, whether it is a time-only or a time-and-cost ground under your contract.
What prolongation cost consists of
- Site establishment: offices, camps, utilities, security for the extended period.
- Idle plant and equipment: owned or hired, at documented rates, for documented idle days.
- Staff and supervision retention: the site team kept on for longer.
- Other direct costs: insurances, bonds, and similar time-related items.
- Head office overhead and profit: the contested head, argued through Hudson, Emden, or Eichleay style apportionment and requiring proof of lost opportunity.
Each needs records that were kept during the extended period: hire dockets, daily plant returns, payroll, invoices. Cost claimed without contemporaneous cost records is the second most common failure after late notice.
When the delay was compensable but the cost period is disputed
The extended period for cost purposes is not always the same as the EOT granted. Cost is recoverable for the period the compensable event actually caused the Contractor to remain on site, which may be earlier or later in the programme than the end-date extension suggests. This is where a delay analyst and a quantum expert work together, and where a claims tool should stop at organising the inputs.
What this means in practice
- Tag every event in the register as time-only or time-and-cost under your contract.
- Argue time and cost in separate sections, with separate proof.
- Keep plant, staff, and establishment cost records daily during any extended period.
- Expect concurrency to attack the money claim even where time is granted.
The DraftMyEOT claim wizard keeps a cost reservation by default and, when you switch on the prolongation cost step, records line items per head with the overhead method left for your expert to choose.
Start a 48-hour draft →Sources
This article is general information about how these contract mechanisms typically work. It is not legal advice, and it is not a substitute for review of your specific contract by a qualified professional.