An extension of time (EOT) claim is a contractor’s formal request to move the contractual completion date because a delay outside its control — a “Relevant Event” under JCT or a “compensation event” under NEC — has pushed the critical path back. A valid EOT gives the contractor relief from liquidated damages for the extended period and keeps the completion date enforceable. Get the notice, causation and delay analysis right and you protect both time and money; get them wrong and you risk losing the claim — or handing the employer an argument that time should be “at large”.
In short: An extension of time (EOT) is a contractual mechanism that moves the completion date when a delay outside the contractor’s control — a “Relevant Event” under JCT or a “compensation event” under NEC — pushes back the critical path. A valid EOT relieves the contractor of liability for liquidated damages for that period, but it normally depends on serving notice within the contract’s timescales. Where the employer causes delay and no EOT is available, the prevention principle can set time “at large”.
This guide explains how EOT claims actually work under the two contracts that dominate UK construction — the JCT and NEC suites — and how the courts have shaped the law on the prevention principle, concurrent delay and notice. It is written for contractors, employers, project managers and quantity surveyors who need a practical, current (2026) understanding before a dispute crystallises into adjudication.
- An EOT relieves the contractor of liquidated damages for the excused period and preserves the employer’s right to deduct LADs for any residual culpable delay.
- Under JCT, entitlement turns on a closed list of Relevant Events (time) and Relevant Matters (money); under NEC there is no separate EOT — delay is dealt with through compensation events that move the Completion Date.
- The prevention principle stops an employer profiting from its own delay, but North Midland v Cyden Homes [2018] confirmed parties can lawfully allocate concurrent-delay risk by contract.
- Notice is often a condition precedent — miss it and you may lose entitlement even where the delay was genuine, especially under NEC’s eight-week time bar.
- Delay-analysis method (time impact analysis, as-planned vs as-built) and float ownership frequently decide the size of the award.
- Most EOT disputes are resolved by statutory adjudication under the Construction Act 1996, not in court — a fast, binding-until-final route.
- What we see in practice
- EOT and money claims travel together: a slipped programme almost always raises a parallel payment or loss-and-expense question.
What an extension of time is — and why it matters
An extension of time is a contractual mechanism that adjusts the date by which the contractor must reach practical (or “substantial”) completion. Standard-form building and engineering contracts fix a completion date and then attach liquidated and ascertained damages (LADs) — a pre-agreed daily or weekly sum the employer can deduct for late completion. The EOT clause is the safety valve: where completion is delayed by something the contract treats as the employer’s risk, the contractor can have the completion date moved forward so that LADs do not bite for that period.
EOT therefore does two jobs at once. First, it provides relief from liquidated damages: every day of validly excused delay is a day for which no LADs are payable. Second, it keeps the LAD machinery alive — because if an employer-caused delay had no route to an EOT, the contractor could argue the employer had “prevented” timely completion and that time should be set “at large”, with the fixed date gone and the LAD clause unenforceable. A well-drafted EOT regime therefore protects the employer’s right to LADs as much as the contractor.
Crucially, an EOT is about time, not automatically about money. Being granted six weeks does not, by itself, entitle the contractor to six weeks of prolongation cost. The money entitlement runs on a separate track — loss and expense (JCT) or the change to the Prices (NEC) — and depends on whether the delaying event is also a compensable one.
Relevant events, relevant matters and compensation events
How you frame an EOT claim depends entirely on the contract. The JCT and NEC suites take fundamentally different approaches.
JCT: Relevant Events (time) and Relevant Matters (money)
Under the JCT family — including the Design and Build 2024 edition now in common use — entitlement to an EOT arises only where the delay is caused by a listed Relevant Event. These are the events the contract treats as the employer’s time risk. Typical Relevant Events include Changes (variations) instructed by the employer, instructions, impediment or default by the employer, exceptionally adverse weather, force majeure, statutory undertakers’ works, civil commotion or terrorism, and changes in statutory requirements. The JCT 2024 edition also brought epidemics and the discovery of asbestos, contaminated material and unexploded ordnance expressly within the Relevant Events list.
Running alongside Relevant Events are Relevant Matters — the narrower list of employer-risk events that entitle the contractor to loss and expense (money), such as prolongation and disruption costs. The distinction matters: exceptionally adverse weather is a Relevant Event but not a Relevant Matter, so a weather delay buys the contractor time but not money. Some JCT 2024 events (such as epidemics or a change in law) are Relevant Events automatically but only become Relevant Matters if the parties select that option in the Contract Particulars — meaning the default is “time, but not money”.
NEC: there is no EOT — only compensation events
The NEC4 Engineering and Construction Contract works very differently. There is no separate “extension of time” procedure at all. Instead, both time and money flow from a single integrated mechanism: the compensation event. Clause 60.1 lists the events — the standard ECC sets out 21 — that entitle the contractor to an assessment that can change both the Prices and the Completion Date (and any Key Dates). There is no end-of-job global “EOT”: each delay must be tied to a specific compensation event and assessed, ideally, at or around the time it arises. NEC’s model is prospective — you forecast the impact forward from the event — and is tightly coupled to the contractor’s regularly updated programme and the early-warning regime.
| Feature | JCT (e.g. DB 2024) | NEC4 ECC |
|---|---|---|
| Time mechanism | Extension of time for “Relevant Events” | Change to Completion Date via “compensation events” |
| Money mechanism | Separate loss & expense for “Relevant Matters” | Same compensation-event assessment changes the Prices |
| Time and money | Two separate tests and lists | Integrated — assessed together |
| Assessment basis | “Fair and reasonable” EOT; often retrospective in practice | Prospective forecast at the time of the event |
| Notice trigger | Notify delay as soon as it becomes reasonably apparent | Notify within 8 weeks of becoming aware (cl 61.3 time bar) |
| Late notice | Employer still fixes a fair completion date at review | Right to time/money for that event can be lost entirely |
The prevention principle and “time at large”
If the employer causes delay (for example by late instructions or variations) and the contract has no mechanism to extend the date for that cause, time can be set “at large” — the contractor then only has to finish within a reasonable time and fixed completion-date damages fall away. This is the prevention principle, illustrated in Peak Construction (Liverpool) Ltd v McKinney Foundations Ltd (1970) 1 BLR 111. A properly drafted extension-of-time clause that covers employer-risk events is what keeps the completion date, and any liquidated damages, intact.
The prevention principle is one of the oldest doctrines in construction law: a party cannot insist on performance of an obligation it has itself prevented the other from performing, and cannot take advantage of its own wrong. So an employer who causes delay — late access, late information, extra work — cannot then enforce the original completion date and deduct LADs as if nothing had happened.
The leading authority is Peak Construction (Liverpool) Ltd v McKinney Foundations Ltd (1970). The employer’s own delay in giving instructions delayed the works, but the EOT clause did not cover that type of employer delay. The Court of Appeal held that, because the contract gave no route to extend time for the employer’s own act of prevention, time was set “at large”: the fixed completion date fell away, the contractor’s obligation became to finish within a reasonable time, and — critically — the employer’s right to liquidated damages was lost. Multiplex Constructions (UK) Ltd v Honeywell Control Systems Ltd later restated the same essence: the promisee cannot insist on an obligation it has prevented the promisor from performing.
This is why every modern standard form contains a broad EOT clause that expressly captures acts of prevention (impediment, default, variations instructed by the employer). The practical lesson is counter-intuitive: a generous EOT clause protects the employer, because it provides the contractual route to extend time that keeps the LAD clause alive and prevents time going at large.
Concurrent delay after Walter Lilly and Cyden Homes
Where two delays run at the same time — one the employer’s risk and one the contractor’s — the starting position from Walter Lilly & Co Ltd v Mackay [2012] EWHC 1773 (TCC) is that the contractor may still recover an extension of time for the employer-risk event. However, in North Midland Building Ltd v Cyden Homes Ltd [2018] EWCA Civ 1744 the Court of Appeal confirmed that parties are free to allocate the risk of concurrent delay by express clause, and such a clause will be enforced. The contract wording, not a default rule, decides the outcome.
Concurrent delay arises where two delaying events operate at the same time and both are effective causes of delay to completion — one an employer-risk event (a Relevant Event), the other a contractor-risk event (the contractor’s own default) — and both bite on the critical path. It is one of the most heavily litigated areas in construction, because the question “who pays for the overlap?” can be worth millions.
In Walter Lilly & Company Ltd v Mackay [2012] EWHC 1773 (TCC), Mr Justice Akenhead — then head of the Technology and Construction Court — held that, on a standard JCT form, where a Relevant Event and a contractor-culpable delay are truly concurrent, the contractor is entitled to a full extension of time for the period of delay caused by the Relevant Event, and is not to have its EOT reduced merely because its own delay ran in parallel. In the dispute over a £5m+ luxury house in West London, the contractor was awarded an EOT to the date of practical completion and around £2.3m of loss and expense. This is sometimes called the “Malmaison approach”: for time, concurrent contractor delay does not defeat the EOT.
That, however, is the default common-law position — and it can be displaced by clear words. In North Midland Building Ltd v Cyden Homes Ltd [2018] EWCA Civ 1744, the JCT contract had been amended so that, where contractor delay was concurrent with a Relevant Event, the concurrent period was not taken into account when assessing the EOT. The contractor argued this offended the prevention principle; the Court of Appeal disagreed and upheld the clause. Parties are free to allocate the risk of concurrent delay as they see fit, and the prevention principle does not override an express, freely agreed allocation. The practical upshot for 2026 is stark: read the concurrency wording first. A bespoke amendment can lawfully shift the risk that Walter Lilly would otherwise place on the employer.
| Scenario | Default position (Walter Lilly) | With a Cyden-style concurrency clause |
|---|---|---|
| Relevant Event only, on the critical path | Full EOT for the delay period | Full EOT — unchanged |
| Contractor delay only, on the critical path | No EOT; LADs run | No EOT; LADs run |
| True concurrent delay (both critical) | Contractor gets full EOT for the time element | Concurrent period excluded — no EOT for the overlap |
| Prolongation money in concurrency | Often reduced/apportioned even where EOT granted | Typically no loss & expense for the concurrent period |
Notice requirements and conditions precedent
An EOT entitlement is only as good as the notice that supports it. Standard forms require the contractor to notify delay promptly and to provide particulars — the cause, the Relevant Event or compensation event relied on, and the estimated effect on completion.
Under JCT Design and Build 2024, the contractor must notify the employer of any delay to progress, with particulars and an estimate of the expected delay, as soon as it becomes reasonably apparent. The 2024 edition tightened the machinery: the employer must, within 14 days, say what further information it needs, and must then notify its EOT decision within eight weeks (reduced from 12 in earlier editions). While JCT’s drafting and the surrounding case law mean the employer generally retains a duty to fix a fair completion date at the final review even if interim notices were imperfect, treating notice as optional is dangerous — late or vague notices weaken causation, invite the employer to allege prejudice, and (where the contract is amended to make notice an express condition precedent) can defeat the claim outright.
Under NEC4, the position is far harsher. Core clause 61.3 requires the contractor to notify a compensation event within eight weeks of becoming aware of it. If the contractor notifies outside that window — and the event is not one the Project Manager was required to notify — the contractor loses the right to any change in the Prices, the Completion Date or Key Dates for that event. The bar is binary: there is no discretion to weigh how late or how genuine the claim is, and an early warning, a site-meeting comment or a progress-report reference will not count — the notification must be a formal, written communication that identifies the event as a compensation event. Courts treat such clauses as conditions precedent and enforce them strictly.
- Relying on a programme update, RFI or site-meeting minute instead of a formal, clearly-labelled notice.
- Missing the NEC eight-week clock from the date of awareness (not the date the effect is felt).
- Notifying the delay but never substantiating the causal link to the critical path.
- Failing to update particulars as the delay’s effect changes.
Delay analysis methods
Once notice is in, the battle moves to causation: did the event actually delay completion, and by how much? That is answered with delay analysis. The main methods recognised in UK practice and by the Society of Construction Law (SCL) Delay and Disruption Protocol (2nd edition) are:
- Time Impact Analysis (TIA) — a prospective, “forecast” method that inserts the delay event into a programme updated to just before the event, to model its likely impact on completion. The SCL Protocol favours assessing the prospective effect of delay contemporaneously using TIA.
- As-planned vs as-built — a retrospective comparison of the baseline programme with what actually happened, to identify and explain the variances. Simple but vulnerable to over-simplification.
- Impacted as-planned — delays added to the baseline; quick but criticised for ignoring actual progress.
- Collapsed as-built (as-built but-for) — delays stripped out of the as-built programme to show what completion would have been without them.
- Windows / time-slice analysis — the critical path examined period by period; often the most robust for complex projects.
The Protocol’s important 2nd-edition shift is that there is no single preferred method where the analysis is carried out long after the event; the right method depends on the records available, the contract terms, the value at stake and the proximity of the analysis to the events. NEC’s prospective philosophy pushes towards TIA in near real time; JCT disputes assessed after the event more often use as-built windows analysis.
Who owns the float?
Float is the spare time in a programme — the amount by which a non-critical activity can slip before it becomes critical and starts to delay completion. Float ownership decides who benefits when an event eats into that slack. There are two broad positions:
- Float is “owned” by the project (the prevailing English view and the SCL Protocol’s general approach): whoever needs it first uses it. So an employer-risk event that consumes float does not entitle the contractor to an EOT until the float is exhausted and completion is actually pushed — because, until then, there is no delay to completion.
- Float belongs to the contractor (sometimes argued, occasionally written into bespoke amendments): any erosion of the contractor’s planned buffer is treated as a delay to be compensated.
In practice, the default English position is that an EOT is only due when an event delays completion, not merely an interim activity with float to spare. A properly prepared, regularly updated and accepted programme is what makes float — and therefore entitlement — measurable at all; where the programme is poor, both sides struggle to prove their case.
Worked example: a programme slippage and the resulting EOT
Scenario — “Riverside Court”, a JCT Design and Build 2024 office refurbishment. Contract sum £6.4m. Contractual completion date: 30 September 2026. Liquidated damages: £8,000 per week. The accepted baseline programme shows the mechanical & electrical (M&E) first-fix on the critical path, with 10 working days of float before the cladding works (also critical) begin.
Event 1 (employer risk): On 6 July 2026 the employer instructs a Change to the riser layout. Redesign and re-procurement delay the M&E first-fix by 4 weeks. The first two weeks are absorbed by float; the remaining two weeks push the cladding start and therefore completion. Critical delay: 2 weeks.
Event 2 (contractor risk): Separately, the contractor’s cladding subcontractor is short-staffed for 1 week during the same period — a delay for which the contractor is responsible — and that week runs concurrently with one of the two critical weeks above.
Notice: The contractor notifies on 9 July 2026 with particulars, identifying the Change as the Relevant Event and its critical-path impact — within JCT’s “as soon as reasonably apparent” requirement.
Assessment (default position, no concurrency amendment):
- The first two weeks of the M&E delay are float — no EOT (no delay to completion yet).
- The remaining 2 weeks delay completion. Of those, 1 week is “pure” employer delay; 1 week is concurrent with the contractor’s own cladding-labour delay.
- Applying Walter Lilly, for the time element the contractor receives a full EOT for both critical weeks despite the concurrency — EOT awarded: 2 weeks. New completion date: 14 October 2026.
- LADs: with a 2-week EOT, the employer cannot levy the £8,000/week LADs for that fortnight — saving the contractor £16,000.
- Money (loss & expense): for the concurrent week the contractor’s prolongation claim is likely to be reduced or refused — it cannot recover prolongation cost for a week it would have lost anyway through its own default — so loss and expense is realistically recoverable for roughly 1 week, not 2.
If the contract had a Cyden-style concurrency clause: the concurrent week would be excluded from the EOT assessment, so the EOT would be 1 week, completion would move only to 7 October 2026, and the employer could levy £8,000 of LADs for the second week. Same facts, very different outcome — driven entirely by the drafting.
What we see in practice
In our advisory work, strong EOT entitlements are most often lost on process rather than on the underlying delay. The most common pattern we see is a contractor with a genuine employer-risk delay failing to serve the contractual notice in time, where notice is a condition precedent, and so losing the entitlement entirely. We also see thin, retrospective delay narratives prepared months later instead of contemporaneous records and an updated programme that show cause and effect. Treating the notice clause and the programme as live obligations from day one, not as paperwork for the end of the job, is what turns a fair claim into a recoverable one.
Common EOT mistakes
- Confusing time with money. An EOT extends the date; it does not automatically pay prolongation. Pleading them as one claim invites a global-claim challenge.
- Ignoring the concurrency wording. Assuming Walter Lilly applies when a bespoke Cyden-style clause has shifted the risk.
- Weak or late notice. Especially fatal under NEC’s eight-week clause 61.3 time bar.
- No contemporaneous programme. Without a regularly updated, accepted programme, critical-path and float arguments collapse.
- Asserting “time at large” too readily. Modern EOT clauses are drafted precisely to capture employer prevention; the argument rarely succeeds where the clause is wide.
- Treating float as the contractor’s to keep. The default English view is that float is for the project; an EOT only arises once completion is actually delayed.
- Picking the wrong delay-analysis method for the records and the timing of the assessment.
Resolving EOT disputes by adjudication
Most EOT and associated loss-and-expense disputes are resolved by adjudication, the fast statutory process available under the Housing Grants, Construction and Regeneration Act 1996. An adjudicator’s decision is binding and enforceable unless and until overturned in litigation or arbitration, so getting the delay narrative and records right before referral matters a great deal.
Most EOT disputes never reach court. Section 108 of the Housing Grants, Construction and Regeneration Act 1996 (the “Construction Act”) gives every party to a construction contract the right to refer a dispute to adjudication “at any time”. The adjudicator must reach a decision within 28 days of referral (extendable by up to 14 days with the referring party’s consent, or longer by agreement), and that decision is binding until the dispute is finally determined by litigation, arbitration or agreement. Parties cannot contract out of this right; if the contract’s own procedure does not comply, the statutory Scheme for Construction Contracts applies.
Adjudication is well suited to EOT and delay claims: it is fast, it keeps cash flowing, and an adjudicator can decide entitlement, quantum of time and the related money in one reference. Enforcement of an adjudicator’s decision, if the losing party will not pay, is then a separate, court-based step in the Technology and Construction Court. For most parties, a well-prepared adjudication is the commercial centre of gravity in any serious EOT dispute.
How Hayhills can help
Hayhills Legal Advisory supports contractors, sub-contractors and employers across the full life of an extension-of-time claim. As a non-reserved legal advisory practice, we act directly on the work that wins or defeats EOT claims: reviewing your JCT or NEC contract and its concurrency and notice provisions; building a compliant notice and particulars strategy; pressure-testing your delay analysis and float arguments; and presenting or defending the claim through adjudication and other ADR. We help you protect time, resist or justify liquidated damages, and keep cash flowing. Where a dispute needs court enforcement of an adjudicator’s decision, an insolvency step, or formal litigation, we advise on strategy and introduce and coordinate a regulated solicitor to carry out the reserved court work — so you have one joined-up team.
Facing a delayed programme or an EOT dispute? Speak to the Hayhills construction advisory team before notices are missed or LADs are deducted. Call 0203 581 5789 or visit our contact page for a confidential, commercially-focused discussion.
Frequently asked questions
What is an extension of time claim in construction?
It is a contractor’s formal request to move the contractual completion date because a delay that the contract treats as the employer’s risk — a Relevant Event under JCT or a compensation event under NEC — has delayed completion. A valid EOT relieves the contractor of liquidated damages for that period.
Does an extension of time mean I get paid extra money?
Not automatically. An EOT only adjusts time. Money for prolongation runs on a separate track — loss and expense (a JCT “Relevant Matter”) or a change to the Prices (an NEC compensation event). Some delays, such as exceptionally adverse weather under JCT, give time but not money.
What is the difference between a Relevant Event and a Relevant Matter?
Under JCT, a Relevant Event entitles the contractor to an extension of time, while a Relevant Matter entitles it to loss and expense. The lists overlap but are not identical — for example, exceptionally adverse weather is a Relevant Event but not a Relevant Matter.
How does NEC4 handle extensions of time?
NEC4 has no separate EOT procedure. Delay and cost are dealt with together through compensation events under clause 60.1, which can change both the Completion Date and the Prices. Each delay must be tied to a specific compensation event and assessed prospectively, ideally near the time it arises.
What is the prevention principle?
It is the rule that a party cannot enforce an obligation it has itself prevented the other party from performing, nor profit from its own wrong. In construction, if an employer-caused delay has no route to an EOT, time can be set “at large”, the fixed completion date falls away and liquidated damages may be unenforceable.
What does “time at large” mean?
It means the fixed completion date no longer applies and the contractor must simply finish within a reasonable time. It usually arises where the employer has prevented completion and the EOT clause cannot cover that delay. A key consequence is that the employer typically loses its right to liquidated damages.
How is concurrent delay treated under English law?
Following Walter Lilly v Mackay [2012], the default position is that where an employer-risk event and a contractor delay are truly concurrent, the contractor still gets a full extension of time for the time element. But North Midland v Cyden Homes [2018] confirmed parties can contract out of this by an express concurrency clause.
Can a contract take away my right to an extension of time for concurrent delay?
Yes. In North Midland Building v Cyden Homes [2018], the Court of Appeal upheld a clause that excluded concurrent contractor delay from the EOT assessment, holding that parties are free to allocate concurrency risk and that the prevention principle does not override a clear, agreed allocation. Always read the concurrency wording.
What happens if I miss the notice deadline for an EOT?
It depends on the contract. Under NEC4 clause 61.3, notifying a compensation event more than eight weeks after becoming aware can lose your right to time and money for that event entirely. Under JCT, late or vague notice weakens the claim and, where notice is an express condition precedent, can defeat it.
How are EOT disputes usually resolved?
Most are resolved by adjudication under section 108 of the Construction Act 1996, which lets either party refer a dispute “at any time”, with a decision within 28 days that binds until finally determined. It is faster and cheaper than court and is the usual route for delay and EOT claims.
This article is for general information only and does not constitute legal or accountancy advice. Hayhills Limited, trading as Hayhills Legal Advisory, provides non-reserved legal advisory services. Always check current requirements at GOV.UK.