Getting up to attic
One suggestion before you go too far with your design would be to talk to the local building inspectors or authorities having jurisdiction. You might be surprised to find that they may frown upon habitable space being accessible only by a folding or removable stairway...and that being the only access. This may be an issue that the local Fire Department may have a problem with for life/safety code reasons.
In most attics, this would be of particular concern because attics usually don't have windows -- no way out if there's a fire.