Is there a limit to the additional information an Employer can request to enable them to make a decision on a notification for acceptance relating to design? Extensive design information has been issued to the Employer to date for the design however they continually request additional information - clarifications on derivations of loading, all possible mitigation measures for risks within the DRA, etc etc. This is leading to a protracted design phase and significant additional works for the designer. The designer is extremely competent in their field, thus the level of their initial design submission is not the question, and have not known requests like this from the Employer (more namely their engineers/designers). Contract form is NEC3 ECC Option A Design and Build.
Firstly check the requirements of your Works Information to see what (if any) constraints the Employer imposed on you in terms of what your design submission should look like and what information they were expecting. If they are now asking for more information than was originally stated then you may be entitled to a compensation event for the provision of additional information.
Your obligations in respect of design submission are at clause 21 and are namely (1) design the parts of the works stated in the Works Information, (2) submit the design as the Works Information requires, (3) do not proceed until the design is accepted, and (4) submit the design in parts if each part can be assessed fully. If there is little or nothing in the Works Information what is reasonable to ask for becomes far more subjective.
I’d suggest notifying this as an early warning and hold a risk reduction meeting to discuss and resolve with the relevant parties. It might help if you can convince them that their actions are unnecessarily delaying progress and also explain that if the design is deficient, correcting it is your risk in the contract, not theirs. You don’t say if X15 has been used but your liability will be different depending on whether it is or not. It could also help to direct them to clause 14.1 i.e. acceptance of the design does not change your liability, they may think they have a duty to approve which they don’t in NEC.
Note that the PM has limited grounds for not accepting the design i.e. it does not comply with the Works Information, therefore it might help to ask them to objectively state which part of the Works Information they think it doesn’t comply with.
Note also that the PM is required to make a decision on accepting the design or not within the period for reply, unless another period is stated in the Works Information. If they fail to do this you again may be compensated cost and time under clause 60.1(6).