Current capacity is 54,000. Original increase was mooted to be 60,000. ARE current is about 9,000 with 6,000 in the lower and 3,000 in the upper - which when you look at the pics is a miracle of cramming people in and on when you see the depth of the stand compared to others. The original idea was to increase by 4,800 to take overall capacity to 58,800 - so an additional 7,800 seats in the new tier, meaning overall capacity for the ARE at 13,800 i.e. to increase over capacity to 62,000 would need another 3,200 seat meaning overall capacity of 17,000. Spurs new 'Kop' is 17,000 but that curves around to the side.
We could potentially go further back with it and the centre part would still be within guidelines but the edges would be the cheap seats unless you did a Kop type arrangement. Not sure how much that would add.
They could off course replicate the overhanging tier that is about to be removed but slightly higher and better constructed and running back then in a continuous tier as already proposed potentially getting your 3,000 seats back with too much extra height.
They are apparently adding in seats this summer in teh Centenary by adding onto the end of each row and narrowing the aisles by one seat - how true this is I dont know but guess if the legislation would allow it. That and the Kop extension of 470 only replaces those GA seats that we lose. We gain disabled places which adds another 200 or so to overall capacity.
That from next year I think, Premier League rules mean away fans have to be located next to the pitch.