It depends on the form: JCT 80 before amendment 15 made the final certificate conclusive evidence of the architect's satisfaction, as Crown Estate v Mowlem confirmed, while amendment 15 and JCT 2011 narrowed that effect, and forms such as ICE, GC/Works/1 and NEC give the certificate no conclusive effect, so the ordinary Limitation Act periods apply, subject to the longer periods for a Defective Premises Act claim after the Building Safety Act 2022

It depends on the form of contract. Under JCT 80 before amendment 15 in July 1995, the final certificate was conclusive evidence of the architect's satisfaction as to materials and workmanship, and Crown Estate Commissioners v John Mowlem held that a claim challenging it was bound to fail without a hearing on the merits, provided any action was brought within 28 days. Amendment 15, whose approach JCT 2011 carries at clause 1.9.1, narrowed conclusiveness to items the contract documents expressly reserve to the architect's approval. In London Borough of Barking & Dagenham v Terrapin the Court of Appeal held that conclusiveness covered latent and patent defects but not defective design. MF/1 and the IChemE Red Book make the certificate conclusive with their own exceptions, MF/1 requiring action within three months. The ICE Conditions, GC/Works/1 and NEC3 give the certificate no conclusive effect, so the ordinary Limitation Act periods, six years for a simple contract or twelve for a deed, apply. Those contractual periods are not the whole story: a claim under the Defective Premises Act 1972 now carries longer periods following the Building Safety Act 2022. Fraudulent concealment can defeat conclusiveness, depending on the clause and the nature of the concealment, as Gray v TP Bennett and William Hill v Bernard Sunley show.