D&B novation: liability before and after contractor appointment

A commercial developer engages you as architect under a RIBA Standard PSC 2020 appointment to take a 40-unit residential-over-retail scheme to RIBA Stage 3. The fee is fixed at £180,000. The client's intention, stated in the appointment letter but not in the PSC itself, is to novate your appointment to the main contractor once the building contract is placed on a JCT Design and Build 2024 basis. At Stage 3, whilst checking the structural engineer's calculations for the transfer slab over the retail units, you identify that the design assumes a concrete compressive strength that the structural engineer's specification does not achieve. The error, if built as specified, would produce a transfer slab with insufficient load capacity for the upper residential floors. You raise the issue with the structural engineer, who denies there is a problem and refuses to revise the calculations. The client is pressing you to issue the Stage 3 package to tender as the programme is already delayed. The contractor has been identified and novation is expected within four weeks. You have not yet been novated. Your RIBA PSC 2020 appointment is still with the original client.