I don't know how much of what I'm going to say is true, but it's what I've always been told when mentioning platdorm lengthening and the costs implied...
Apparently, when
NR» does any work on a platform, then it is consider a station modernization and all the disability access regulations must be followed for the resulting 'new' station. So to put that platform back in service will mean that the whole station loses the old bulding excemptions and the whole station must be made disable accessible.
So that's why NR almost never does any work on any old station platform without lifts or ramps already fitted...
But, as I said, it could all be a lie that somehow makes some sense to me!