Under JCT Intermediate Building Contract 2009, if serious defects in a project become known, but the time for withholding notice has expired, is it legally permissible for an employer to offset the value against the certified amount?

In principle, the most likely legal proceeding is for the employer to pay, and should the defects not be satisfactorily corrected by the next valuation date, the onus is on the architect to certify the amount due accurately by incorporating the defects into the account calculations. In reality, such a situation can lead to legal disputes.