Leaving aside all the complicated stuff about whether you are allowed to
do it and whether the joists are strong enough, I have a question.

Instead of building a room that you have to climb up to, why not build
cupboards that move up and down? That way, the user stays downstairs,
presses a button (say), and his wardrobes descend to him.

The gap between joists is rather small, so you might need to take part
of one out and strengthen the neighbours accordingly.

---
This email has been checked for viruses by Avast antivirus software.
http://www.avast.com