Investigation first and then complain violation duty to complain?

In its ruling last Friday (March 27, 2026), the Supreme Court ruled on the question of whether Hoogheemraadschap van Schieland en de Krimpenwaard (hereinafter HHSK) complained in a timely manner to the plaintiff in cassation (see ECLI:NL:HR:2026:507). It is an important ruling on the complaint duty. Violation of the duty to complain can have far-reaching consequences.

Construction contract for construction of indulgence structure

Between HHSK and the claimant in 2010, a works contract established. Under this agreement, the plaintiff would design and construct a drainage structure for HHSK. A drainage structure is a structure designed to drain excess water. According to the contracting agreement, this drainage structure should have a capacity of 20 m3/s.

Test shows that structure does not have the agreed specifications

The capacity of the spillway construction was tested by HHSK in 2018. It was found that the spillway construction only had a capacity of 6.4 m3/s. HHSK doubted this outcome. It therefore commissioned a third party to investigate the capacity of the spillway construction. On July 1, 2019, HHSK received the conclusion of the study. This conclusion showed that the spillway construction has a lower capacity than the agreed 20 m3/s. HHSK informed the claimant about the conclusion of the investigation in mid-August 2019 (i.e. a month and a half later). Subsequently, the claimant was placed in default by HHSK on October 6, 2020. According to the plaintiff in cassation, HHSK complained too late. In fact, the duty to complain had already started in 2018. HHSK disagrees. It believes that it should have been given the opportunity to conduct further investigation first.

Appeal for breach of duty to complain fails

In the first instance, the claim of HHSK - that the contractor is sentenced to repair the defects in the indulgence structure within a year - rejected. In appeal the plaintiff argued that HHSK's claims should be declared inadmissible as the duty of complaint of article 6:89 BW (see ECLI:NL:GHARL:2024:5613). This appeal by the plaintiff does not succeed. The court rules:

“HHSK tested the capacity of the ARE under controlled conditions in February 2018. The outcome was that the ARE had a capacity of 6.4 m3/s instead of the agreed 20m3/s. (...) Because the outcome of that test surprised HHSK and it doubted whether that outcome was correct, HHSK commissioned Deltares to conduct a desk study for verification purposes that would analyze the capacity of the spillway construction. The final report of this desk study was received by HHSK on July 1, 2019. This report from Deltares shows that Deltares recalculated the capacity at 7.4 m3/s based on the 2018 system test data. Deltares estimates the capacity of the ARE at high level in the Rotte River to be 11 to 12 m3/s. HHSK communicated the results of the Deltares report to [respondent] on August 16, 2019. From this course of events, the court deduces that it first became clear to HHSK as of July 1, 2019 to what extent the capacity of the ARE did not meet the Basic Agreement. The fact that the 2018 system test already contained indications of this does not mean that HHSK should have already informed [the respondent] at that time. HHSK itself doubted the outcome of the system test with respect to the ARE. In the opinion of the Court of Appeal, before proceeding to a drastic step such as reporting a defect to [respondent] years after the ARE was put into service, HHSK was entitled, under those circumstances, to first conduct further investigation. The fact that this further investigation subsequently took more than a year is in itself insufficient to rule that the duty to complain was breached. That [the respondent]”s interests were damaged as a result has not become sufficiently plausible. After HHSK received the report from Deltares, it informed [the respondent] of that report sufficiently energetically and addressed the ARE's too limited capacity in the email of August 16, 2019."

The court set aside the court's ruling and ordered the plaintiff to repair the defects within two years in such a way that the outlet construction achieves a capacity of at least 20 m3/s.

The Supreme Court rejected the plaintiff's appeal and upheld the court's judgment.

Room for investigation before the complaint period begins to run

Despite the fact that on the basis of the obligation to complain it is wise to notify the other party of defects or inaccuracies in a contract as soon as possible, it can be deduced from the Supreme Court's judgment that there may be a certain amount of time for the complainant to investigate the defects. After all, this judgment shows that - under circumstances - it is possible to have an expert investigation carried out first, before a complaint is made to the other party and before the term of the obligation to complain starts to run.

So when does the duty to complain begin?

However, the ruling shows that not every suspicion that something might be wrong already leads to the obligation to complain. Complaining, the court said in the ruling upheld by the Supreme Court, is a drastic step. The person who discovers a defect may investigate before having to complain.

The fact that this investigation takes a long time may be relevant to the question of whether the duty to complain runs, but by itself not sufficient for the conclusion that the complaint was made too late. Nevertheless, as a client, you take a risk if you do not complain to the other party immediately after you suspect a defect. This is because the law states that the obligation to complain starts to run not only at the moment you are aware of the defect, but also at the moment you ‘ought to be. Therefore: complain in time. Even if - as the court says - that is a drastic step.