Clause 18.1 of NEC3 ECC and clause 17.2 of NEC4 ECC say you notify the other as soon as you become aware of anything which requires you to do something that is illegal or impossible and that if the PM agrees, he or she changes the Works Information or Scope appropriately, which would probably be a compensation event.
Even if the PM does not agree that it is impossible, it sounds as if (a) this is an instruction from the PM for a change, so is a compensation event and (b) if it is very hard to do, but not quite impossible, it will probably be expensive to do and the Employer/Client will ultimately pay for that. So you could early warn !
Even if the PM does not agree that it is impossible, it sounds as if (a) this is an instruction from the PM for a change, so is a compensation event and (b) if it is very hard to do, but not quite impossible, it will probably be expensive to do and the Employer/Client will ultimately pay for that. So you could early warn !