EmmyLee,
I have never seen second floor plans when the concert hall existed, but here is the plan of the first floor:
The Concert Hall is the large double room just before Alexandra's Formal Reception Room and the Small Library.
As I underdstand it the Concert hall was the full height of the first floor and mezzanine, so it would have been the same height as The Maple room and Nichollas' New Study including the balconies. I do not believe it would have gone into the second floor at all and I imagine the floor plan of the second floor would look fairly similar to the way it looks today. I thinks most of the changes made by N&A were adding internal walls to make the larger rooms into several smaller ones. The left side of the second floor was probably very similar to the right side as that side wasn't renovated by N&A as far as I know and would be an clues as to what the Children's floor might have originally looked like.
This plan came from the section of the Alexander Palace Time Machine under the AP Palace Tour/Plans, Maps and Churches section where there is a link to
Designs for the Alexander Palace by Quarenghi. Unfortunately, this link seems not to be working.
BobG