Yesterday I installed about 35m2 of dry flooring on the top floor. Each piece was 2.5 cm thick, 120cm X 60cm. Each one weighed a little over 22kg. It was a huge pain in the ass, but we got it done. I was really sore this morning.
When the room is done, I will either put up a wall across the room flush with the hole where the stairs will come up or I will put up walls around the stairs with a door at the top. Either way there needs to be some sort of separation for when I make loud music.