No different to any other countries capital projects . Projects of that scale and circumstance are rare, hence difficult to estimate without detailed design.
Most projects of a decent scale in the UK use two stage procurement to fix the costs prior to commencement.
Thatâs how two stage is supposed to work. It doesnât mean however that the costs wonât go up massively from stage 1 to 2. The costs of providing a for construction design in advance of deciding to proceed on a project of this scale would be prohibitive.