INTRODUCTION

1. The FIDIC Conditions of Contract for Plant and Design-Build, 1999 (“Yellow Book”) is one of the most widely adopted standard form frameworks in international construction and infrastructure projects. The Yellow Book allocates risk in these projects through a detailed contractual structure.  
2. In a significant judgment on the mechanism of such risk allocation, the UK Privy Council (“UKPC”) in Uniform Building Contractors Ltd v Water and Sewerage Authority of Trinidad and Tobago, [2026] UKPC 2 (“Uniform”) addressed a foundational question: when, and in what circumstances, can works falling outside the original contractual scope constitute a contractual “variation”, and can an entitlement to additional payment arise in the absence of compliance with the procedure prescribed in the Yellow Book? 

2. BACKGROUND

3. In the present case, Uniform Building Contractors Ltd. (“Contractor”) entered into a lump sum design-and-build contract for pipeline works (“Contract”) with the Water and Sewerage Authority of Trinidad and Tobago (“Employer”). The Contractor agreed to design, supply and install approximately 28.43 km of pipeline from Rio Claro to Mayaro in Trinidad for a combined lump sum value exceeding TT $28 million. The Contract incorporated the terms of the Yellow Book.  
4. Following disputes that arose during the performance of the Contract, the Employer terminated the Contract by notices issued in May and June 2009. The Contractor alleged that it had performed additional works outside the scope of the Contract, which it characterised as variations of the Contract. Accordingly, it instituted proceedings before the High Court of the Republic of Trinidad and Tobago (“HC”), claiming additional payment for four alleged work variations: (i) laying pipelines in the roadway rather than verges; (ii) removal of unsuitable material; (iii) importation of backfill; and (iv) night work.  
5. The HC dismissed the claim. The Court of Appeal of the Republic of Trinidad and Tobago (“COA”) reversed that decision on the basis that the Contractor’s performance of such contested works had been approved by the Engineer on site, and that consequently the procedural requirements of raising variations had effectively been waived. Unsurprisingly, the Employer sought to appeal this decision before the UKPC.  

3. THE PARTIES’ SUBMISSIONS   

    The Contractor’s Submissions  

6. At the outset, the Contractor attempted to argue that since it had commenced prior to the Contract being finalised, and that neither itself nor the Employer had had sufficient time to carry out investigations of the site, the full effect of the provisions of the Contract ought not to be enforced against it.
7. The Contractor asserted that the contested works amounted to variations of the Contract under Clause 13.1 (Right to Vary) of the Yellow Book which required a variation to be “initiated by the Engineer” either by an express instruction or a formal request for a proposal. The Contractor argued that these works had arisen from directions imposed by the Engineer on site and fell within the types of changes contemplated under Clause 13.1, including alterations to the scope, sequence, method, or timing of execution of the Works.  
8. The Contractor emphasised that the Contract operated flexibly with the Engineer approving disputed items on site by way of ongoing discussions and practical decisions without insisting on formal written procedure. Such conduct constituted instructions within Clause 3.3 (Instructions of the Engineer) and could amount to variations under Clause 13.1 since they altered the definition of “Works” in the Contract. 
9. Accordingly, the Contractor sought additional payment for these contested works by relying on Clause 13.3 (Variation Procedure) and Clause 12 (Measurement and Evaluation) on the basis that once a variation had been instructed, it fell to be evaluated in accordance with the terms of the Contract. 
10. The Contractor further argued that even if it had failed to comply with the procedural requirements of the Contract, on the basis of waiver and estoppel principles, it was unfair and inequitable for the Employer to have the benefit of the contested works without the Contractor receiving any payment for them.
    
    The Employer’s Submissions 
11. The Employer resisted the Contractor’s claims on the basis that the requirements of what constituted a variation under Clause 13.1 remained unfulfilled. It argued that these contested works had already been expressly or impliedly included within the definitions of “Work”, “Employer’s Requirements”, and “Bill of Quantities” as contained in the Yellow Book. In any event, the Employer contended that no necessary qualifying initiation within the meaning of Clause 13.1 had occurred in respect of these contested works. 
12. The Employer argued that the Engineer’s conduct on site – by general discussions and/or practical decisions – did not fulfil the rigorous criteria for a work to qualify for a variation under the terms of the Contract. The Employer asserted that not every instruction issued by the Engineer at site amounted to a variation and only those instructions which altered the definition of “Works” in the Contract could properly be construed as a variation under Clause 13.1.  In the present case, the definition of “Works” remained unaltered by the Engineer’s instructions at sit with the result that there was no justification for any variations to the Contract.  
13. Accordingly, the Employer challenged the Contractor’s claims for additional payment on the basis that the contested works had been included in the lump-sum packages of the Contract and could not attract additional payment merely because they had proved to be more onerous to the Contractor during execution.  

4. THE SIGNIFICANCE OF THE DECISION OF THE UKPC 

14. The UKPC dismissed the Contractor’s claim in full and reversed the decision of the COA. 
    
    The nature and terms of the Contract 
15. The UKPC rejected the Contract’s argument that it ought not to be strictly held to the Contract, for several reasons. Upon assessing the facts, the UKPC found that the Contractor had sufficient time in two separate phases to assess the site: first, two months between receiving the tender invitation and putting in their bid; and second, four months between putting in their bid and receiving the award. The UKPC affirmed that that the Contractor was bound by the terms of the Contract that it freely agreed to, having had adequate opportunity to survey the site and price the lump sum works. It observed that there was no legal basis for the Contractor’s attempt to re-allocate risk and reward between itself and the Employer, contrary to the Contract.

    The characterisation of contested works done as variations to the Contract 

16. The UKPC observed that as the HC had failed to evaluate all the evidence before it while determining whether the contested works amounted to variations, its assessment could not be characterised as a complete finding on facts. Therefore, it fell to the COA and consequently the UKPC to undertake its own assessment on whether the four contested works amounted to variations under the Contract. 
17. The UKPC held that the issue of whether the contested works done amounted to variations to the Contract within the meaning of Clause 13.1, turned on the proper construction of the Contract, including the Yellow Book which formed part of the Contract. The UKPC observed that neither the High Court nor the Court of Appeal had analysed whether the contested works fell within the definitions of “Works” or “Employer’s Requirements” in the Contract.  
18. Noting that Clause 1.1.6.9 defined a variation as “any change to the Employer’s Requirements or the Works which it is instructed or approved as a variation under the Clause 13”, the UKPC found that none of the contested works amounted to variations. This was partially because the general terms of the Yellow Book indicated that the price of carrying out these contested works ought to have been included within the lump sum price of the Contract; and partially because various specific terms of the Contract itself “expressly or impliedly” covered these contested works.
    
    The Contractor’s procedural failures relating to the Engineer’s conduct 
19. The UKPC was unpersuaded by the Contractor’s argument that the Engineer’s conduct on site constituted oral instructions that amounted to variations under Clause 13.1. The UKPC observed that the Engineer’s authority to sanction variations was circumscribed by the explicit requirement in Clause 3.3 (Instructions of the Engineer) for such instructions to have been issued in writing for these to amount to any variation.  
20. Further, the UKPC identified two significant procedural failures in the Contractor’s conduct. 
21. First, finding that the absence of any written instructions was immaterial, the UKPC remarked that even assuming that the Engineer had orally instructed the Contractor to carry out any variation, the Contractor would have had to provide the Engineer with a formal proposal, including notifying him of the additional cost per Clause 3.6 specifically introduced in the Contract and seeking a determination of the value of such variations per Clause 3.5 (Determinations) of the Yellow Book. It is this determination by the Engineer that could give rise to the Contractor’s entitlement for additional payment in respect of the contested works. The UKPC noted that the Contractor’s first procedural failure arose from its failure to give early and proper notice of the increased costs associated with the contested works and the concomitant failure to seek a determination from the Engineer. In the absence of any such determination – and of any request for such determination – the Contractor could not be entitled to any additional payment.  
22. Second, and fatally, the UKPC observed that the Contractor had altogether failed to raise a notice of claim concerning the contested works within the 28-day timeline stipulated under Clause 20.1 (Contractor’s Claims). The UKPC held that the language of Clause 20.1 was “in classic condition precedent form” inextricably linked to payment for the Contractor’s claims. The UKPC found that if the Contractor had wanted to claim payment for the contested works and they could not do so as a result of the Engineer’s failure to play his part in the contractual process, it was up to them to notify the Employer of a claim within the 28-day period in Clause 20.1. Having failed to comply with this condition precedent, the Contractor was prevented by the Contract from making any claim in respect of the alleged variations and/or the Employer was discharged from any liability of payment in respect of these.  
23. In respect of the second failure, the UKPC did not accept the COA’s reasoning that Clause 20.1 did not apply to the Contractor because the Contract itself was subsequently terminated. The UKPC did not find that Clause 20.1 could be restricted so easily. First, and noting that termination operated prospectively and not retrospectively, the UKPC held that the Contract governed the conduct of the parties up to termination, and that termination did not wipe out those rights and obligations already accrued. Second, the COA entirely ignored the 28-day period contemplated in Clause 20.1. The subsequent termination of the Contract could not in law resurrect claims that had not been made in time and were therefore no longer open to the Contractor to make at present.
    
    The lack of any waiver, estoppel or fairness considerations 
24. The UKPC overturned the COA’s acceptance of the Contractor’s argument, that on the basis of waiver and estoppel principles, it was unfair and inequitable for the Employer to have the benefit of the contested works without paying for them. The UKPC noted that the Contractor had made no case on waiver or estoppel before the HC and had only raised it for the first time before the COA. Observing that this argument had been raised far too belatedly, the UKPC noted that should the Contractor have wished to rely on waiver and estoppel in the case, it ought to have properly this with necessary evidence, such that the Employer was aware from the outset of the case it would have to meet and that disclosure could have been properly undertaken. Without either proper pleadings or evidence in the present case, the UKPC found that this contention ought not to have been permitted to proceed before the COA, much less for the COA to have overturned the HC’s decision on the basis of such a consideration of alleged “fairness”.  
25. In any event, the UKPC found that the basis on which the COA accepted the Contractor’s argument of waiver and estoppel, failed to give full and consistent effect to the Contract. It dismissed the Contractor’s argument that the Engineer had acted as the Employer’s agent, as being too broad and ignorant of the express constraints on the Engineer’s authority as contained in Clause 3.1 (Engineer’s Duties and Authority). Moreover, it noted that one of the key errors by both the Contractor in making this argument and the COA in accepting it, was to equate the Engineer with the Employer. Any successful case on waiver and estoppel would have had to be based on the conduct of an authorised representative of the Employer who had the power to amend the Contract to include the contested works as variations – this could not have been done by the Engineer.  

5. CONCLUSION

26. In Union, the UKPC confirmed three positions of legal significance. First, the question of whether work constitutes a contractual variation is determined by the contractual definitions in the Yellow Book and not simply by the fact that the work proved more onerous in execution. Second, an entitlement to additional payment under the Yellow Book depends on compliance with the proper contractual requirements. Third, informal conduct on site cannot readily be treated as dispensing with contractual requirements absent a properly established case of waiver and estoppel.  
27. This ruling is a pointed reminder that under the Yellow Book, entitlement to payment flows from strict contractual compliance. In disputes concerning alleged contractual variations, questions of scope, procedure, and authority remain determinative with the result that where the agreed procedure is not followed, claims for payment will not survive. The UKPC has clarified in no uncertain terms that under the Yellow Book, commercial practicality does not override a construction contract and instead, must operate through it.