4000 upper tier south stand is 1.6 times the north stand's 2500. The main stand is about 3500. So 1.6 times 3500 is 5600 upper tier. 20,200+5600=25,800.
If the main stand was to be knocked down and rebuilt to the same size as the new south stand then that's about 10,000 and another 1000 ish in the corner. So, about 26,000.
Getting planning for that will be Fun! Fun! Fun!