Law: late discovered defect, limitation periods, and the negligence test

In May 2018 you completed Practical Completion on the £3 million refurbishment of a four storey commercial office building in central Bristol. The appointment was a RIBA Standard Form of Agreement 2010, executed as a deed in 2015. The Stage 4 drawings included a parapet detail you copied verbatim from a leading proprietary single-ply roofing manufacturer's published catalogue. The catalogue showed the detail as a "tested" condition. The contract was a JCT SBC 2011 Without Quantities. You were the architect and CA. In April 2026 the building's owner, a property investment trust who took ownership of the building in 2019, notices staining on internal soffits at fifth floor level. They commission a building survey. The surveyor concludes the parapet capping detail is defective: the upstand height between the membrane termination and the parapet coping is below manufacturer's current recommendation, the membrane is not adequately bonded, and water has been tracking down the inside face of the parapet for at least four years, causing concrete spalling, corrosion of structural reinforcement, and timber decay in adjacent internal joinery. Repair costs are estimated at £620,000 plus £140,000 of consequential business interruption to two tenants. On 6 May 2026 the owner's solicitors send you a Letter of Claim under the Pre-Action Protocol for Professional Negligence alleging professional negligence in the design of the parapet detail. They quantify damages at £760,000. Your professional indemnity policy is on a claims-made basis with a £1 million limit and a £25,000 excess. You have not notified insurers. The original drawings remain in your archive. The single-ply manufacturer's 2015 catalogue, from which you copied the detail, is also available; the manufacturer has since revised the published detail twice.