Split review - extend deadline?

Dear all, A quick question: I am working on a C&S project - aiming at a split review (construction phase & design phase). Since it's a rather large and complex project, the construction phase will be longer than the 18 months between both reviews (as stated in the certifcation documentation). One thing we could do: submit the design phase 18 months before substantial completion (months after the actual construction has already started), but that seems a bit strange to not have the review phases run parallel to the actual design and construction phase. Does anybody know if there are possibilities to extend the 18-month deadline? thanks!