Rising temperatures, rising risk: What extreme heat means for construction projects 

By the middle of July 2026, the UK had already lived through three official heatwaves this summer. Norfolk recorded 37.7 degrees in June, London touched 35 in a fresh spell in July, and the country had by that point seen more days above 30 degrees than the whole of 1976. It was also the first year on record to reach 35 degrees in each of May, June and July. This followed a 2025 summer that was itself the warmest on record for the UK. What was once treated as an occasional anomaly, worth a headline and a few days of disrupted commutes, has become the default pattern of the British summer. For an industry whose contracts, methods and infrastructure were designed around a cooler and more predictable climate, this shift raises real and immediate legal questions.

Much of the UK’s built environment, and much of the legal framework that governs how it gets built, assumes a temperate climate that is becoming harder to rely on. Contractors, developers, employers and their professional teams are having to grapple with heat not as an occasional inconvenience but as a genuine project risk, one that touches programme, cost, safety and design in ways the standard forms were never built to answer cleanly.

For developers and investors, the consequences extend well beyond the construction phase itself. Delays to completion can put funding drawdown schedules under pressure, create difficulties with pre-let obligations, and complicate practical completion under agreements for lease. A delayed handover can affect the value and marketability of a completed asset just as much as any defect in the works. As buildings are increasingly expected to perform in demanding summer conditions, heat resilience is becoming as much a commercial property issue as it is a construction one, and one that landlords, tenants and funders will want to understand well before completion is due.

Heat as a source of delay and disruption

Heat affects a construction site in ways that are often underestimated until they are experienced firsthand. Productivity falls when workers need more frequent breaks and additional hydration. Certain activities become difficult or impossible during peak temperatures, including concrete pouring and curing, and the application of asphalt, sealants and adhesives, all of which behave differently once ambient and surface temperatures climb beyond their intended working range. Equipment can overheat. Fire risk increases, particularly where dry vegetation sits close to site boundaries or access routes. Health and safety obligations may require work to stop altogether during the hottest parts of the day, even though English law does not currently prescribe a maximum temperature at which outdoor work must cease.

Where these disruptions delay completion, contractors will naturally look to the contract for relief. Whether that relief exists, and how far it extends, depends heavily on which standard form is in use and how it has been amended.

Under JCT contracts, exceptionally adverse weather conditions are listed as a Relevant Event, meaning a contractor who can demonstrate that the threshold is met may be entitled to an extension of time. The difficulty lies in the word “exceptionally”. JCT gives no definition of it, and the limited case law that exists suggests the assessment turns on the weather itself rather than simply its impact on the works, judged against historical patterns for the time of year and location in question. Crucially, exceptionally adverse weather is a Relevant Event but not a Relevant Matter under JCT, so even a successful claim typically secures more time without any accompanying entitlement to loss and expense. As heatwaves become more frequent, the practical question of what still counts as exceptional becomes harder to answer, and the historical weather records parties once relied on to define the threshold are themselves becoming less representative of current conditions.

NEC contracts take a more structured, if still demanding, approach. Weather can only qualify as a compensation event where it is shown to occur less frequently than once in ten years for that calendar month, measured against the weather data stated in the Contract Data. Where a contractor can meet that evidential bar, NEC is more generous than JCT in that it can provide both additional time and additional cost. The trade-off is that the ten-year threshold is a demanding one to satisfy, and parties using NEC on projects with known heat exposure would be well advised to consider amending the weather measurement provisions at the outset rather than relying on the default position.

FIDIC contracts frame the issue slightly differently again. Under the 2017 Red and Yellow Books, a contractor may claim an extension of time where completion is delayed by exceptionally adverse climatic conditions, defined as conditions that are unforeseeable having regard to climatic data made available by the employer or published for the relevant location. This unforeseeability test creates its own tension as extreme heat becomes more common. Conditions that were genuinely unforeseeable a decade ago may now sit within the range that a reasonably informed contractor ought to have anticipated, which could in time narrow the availability of relief just as the risk itself grows more frequent. It is also worth noting that this entitlement does not extend to the FIDIC Silver Book, used for EPC and turnkey projects, where the contractor typically carries far more of the climatic risk from the outset.

Across all three forms, the common thread is that the drafters were working from a climate baseline that no longer reflects reality. Provisions built around the idea of “exceptional” or “unforeseeable” heat become progressively harder to invoke as heat itself becomes ordinary, leaving both employers and contractors with genuine uncertainty about where the risk actually sits.

Establishing entitlement in practice

Even where a contract does provide a route to relief, meeting the contractual threshold is only the starting point. A contractor will still need to show that proper notice was given in accordance with the contract, that any relevant guidance was followed, and that there is a clear and evidenced link between the heat event and the specific delay or disruption claimed. Establishing that link is often the hardest part of the exercise. Rainfall tends to leave visible, easily documented consequences such as flooding or waterlogged ground. Heat is more diffuse in its effects, and without a recognised industry benchmark for when a given temperature makes particular work impossible or unsafe, causation can be genuinely difficult to prove.

This makes contemporaneous evidence essential. Site diaries, weather station data, photographic records, progress reports and properly reasoned delay analyses will usually determine whether a claim succeeds. Where a project is known to carry heat exposure, whether because of its location, its programme timing or the nature of the works involved, it is worth agreeing at the outset how weather data will be recorded and what baseline will be used to measure against it. Early, open conversations between employer, contractor and subcontractors about anticipated heat risk and how it will be managed tend to produce far better outcomes than disputes reconstructed after the event from incomplete records.

Design obligations and the shifting standard of care

Heat risk is not confined to the construction phase. It also sits squarely within the design obligations owed by architects, engineers and other consultants, particularly for residential developments. Since June 2022, Approved Document O has set out overheating mitigation requirements for new residential buildings in England, addressing both the prevention of excessive solar gain through glazing and orientation and the provision of adequate means to remove excess heat once it has built up indoors. Designers now need to turn their minds to whether a building will remain habitable in the kind of summer temperatures the UK is increasingly experiencing, not simply the kind it experienced when the relevant standards were first written.

As climate resilience becomes a more explicit part of what clients and regulators expect, the standard of reasonable skill and care that consultants are judged against is likely to move with it. A design that complied with expectations a decade ago may not necessarily be defensible today if it fails to account for foreseeable heat exposure over the building’s intended life. Where contracts impose outcome-based obligations tied to design life or climate resilience, those obligations deserve particularly careful scrutiny during procurement, since they may sit outside the scope of standard professional indemnity cover and could leave a consultant more exposed than they realise.

Beyond the building envelope itself, prolonged heat and drought bring a further, less obvious risk. Extended dry periods can cause shrinkage in clay soils, affecting foundations and drainage and increasing the likelihood of ground conditions differing from what was anticipated at design stage. Ground investigation carried out during a wetter period may simply not reflect the conditions a project actually encounters once built, which has implications for both design assumptions and any subsequent claims relating to unforeseen ground conditions.

Insurance also deserves careful consideration. Whether contract works, CAR or latent defects policies respond may depend not only on the nature of the damage, but on whether losses are characterised as resulting from heat, drought-induced ground movement or some combination of both. Latent defects cover written some years ago may not contemplate the risks now posed by the increasingly hot climate. Business interruption cover for occupied buildings raises similar questions where extreme heat forces a building to close or operate at reduced capacity. As insurers become more alert to climate risk generally, project teams should expect closer scrutiny of how heat exposure is priced and covered and should not assume that policies drafted for an earlier climate will respond in the way originally intended.

What parties should be doing now

None of this means that heat cannot be managed. It means that the assumptions baked into most standard form contracts, and into many design briefs, were set against a climate that is no longer a reliable guide to what a project will actually experience. Employers, contractors and consultants who want certainty rather than argument after the event should be looking at their contracts before problems arise rather than after.

That starts with reviewing how extension of time and relief provisions are drafted, and considering whether bespoke temperature thresholds, notice requirements and evidential mechanisms would serve the project better than the generic “exceptional” or “unforeseeable” language found in the unamended standard forms. It extends to force majeure, relief event and change in law clauses, which should be checked to see whether they genuinely capture heat-related risk or simply assume it will be caught elsewhere. It also means giving proper thought, at procurement stage, to design obligations connected to climate resilience and design life, and to whether professional indemnity cover actually responds to what is being promised.

Procurement itself deserves closer attention than it typically receives. Tender programmes are often built around historic productivity assumptions that no longer hold on a growing number of summer projects, and employers may need to consider whether working hours should be adjusted for peak heat periods before a programme is even agreed. Pricing should allow realistically for the productivity loss that hot weather brings, rather than treating it as a contingency to be absorbed after the event. Some employers are also beginning to specify their own temperature thresholds and monitoring requirements at tender stage, which removes much of the ambiguity that the unamended standard forms leave behind and gives both sides a clearer basis for pricing the risk from the outset.

Many disputes arising from extreme weather turn out, on closer inspection, to be less about whether heat actually occurred and more about whether the parties communicated effectively as conditions developed. Early warning of an approaching heatwave, collaborative discussion of how the programme might be adjusted, and a genuine attempt to agree mitigation measures before disruption sets in will usually do more to prevent a dispute than any amount of after-the-event analysis. Where those conversations do take place, it is worth documenting them properly, since a written record of what was discussed and agreed at the time is often the difference between a straightforward claim and a contested one.

Standard forms are unlikely to remain static in the face of this. As extreme heat becomes an increasingly predictable feature of UK projects rather than an exceptional one, parties can expect closer scrutiny of how weather risk is allocated, a growing appetite for bespoke amendments, and in time, pressure for the industry standard contracts themselves to be revisited. Heat is no longer an unusual feature of the British summer, and the industry’s contracts, records and design assumptions need to reflect that. The question for anyone involved in a live or upcoming project is not whether the UK’s climate has changed, but whether their own arrangements have kept pace with it.