They are building a huge new by building behind the old stand and then joining the seating up.
I'm not sure what's happening inside to be honest.
I'd love to know.
To use an analogy. When big ships are built, but the work is farmed out to more then one shipbuilding yard, the ship is made in sections from bluprints, down to an exact specification. That way, when two sections of the ship come together from different shipyards, to be joined, all of the bulkheads, doorways, decks etc. all line up perfectly, and just need to be welded together to fit.
My reading of the new mainstand is that all of the newbuild will be built behind the existing, and sometime near the end, they will literally take off the outside skin of the existing main stand, and right behind it will be all of the new buildning already built, and just need joining up.
So say, you might have a corridor in the newbuild that leads nowhere but to the outside brick wall on the outside of the existing, and as soon as you knock the wall through from the existing, there is a ready made corridor there leading to the new facilities etc.
You can build 90% of what you need, without disturbing the day to day matchday running of the existing main stand and facilities. Only near the end has there to be any disruption to dovetail the old with the new.
Thats the way I read it anyway.
I'm sure someone more learned on these things than me, can fill in the blanks.