There may be a number of code/safety issues to consider here.

If you live in an earthquake prone area, I'd be a little nervous about
a freestanding room, especially one that is going to be top-heavy due
to storage and/or guests.

The top is going to have to meet requirements for a "regular" floor,
since it will have to support the dead load of the storage as well as
the live load of 1 or 2 people climbing into bed. So you are going to
need joists, a subfloor and possibly a finish floor. Go to
http://www.awc.org/calculators/span/...rcalcstyle.asp to
determine the joist size based on what you are planning to put on top
of the cube.

By the way, even if the top of the cube is the full 8 x 8, that's
pretty cramped for sleeping area. A single bed is going to take up over
a third of that space, which I guess is OK in an emergency.

I'm also a little confused by 2 seemingly contradicting lines in your
post.

First you say: My...concern is not attaching the room to the existing
structure.
Then you ask: what is the best way to bolt it to the floor?

So, let's assume it is going to be attached, at least at the floor.
Let's also assume you don't want your guests rolling out of bed and
falling to the floor - which means a railing of some type. Consider
extending the corner posts up to the existing ceiling and attaching
them to the joists of the existing structure. This will give you both a
firm attachment to the existing structure and a firm place to attach
your railings to. Seeing that the cube is going to be top heavy, you
are going to want to have some means to prevent racking. Attaching the
cube to the existing ceiling will prevent that. If not, you might want
to consider bracing on the side walls.

While I've never used metal studs, I do know that they do not have any
real strength until the drywall is attached. I can't imagine how they
could be used as the top plate to support the joists you are going to
need to build the floor on top of the cube.

Finally, If you are planning to brace the side walls, you'll need to
use plywood because I wouldn't rely on drywall to provide enough
bracing strength for a top heavy cube.

Check out these pictures to see the effect of wind on a freestanding
structure that wasn't braced properly.
http://www.unified-eng.com/ch/bracing.html I know you won't be dealing
with wind, but I can imagine that an uneven load on the top of a free
standing cube might eventually have the same effect.


wrote:
I want to build an 8x8 room, inside a large room with hardwood floors.
Building the room is OK with landlord. I want to use metal studs, and I
want the room to be a freestanding cube with a floor and a ceiling. I
want to use the ceiling on top of the cube as storage space and a guest
sleeping area, so it needs to support some load.

I've done a bit of construction work before, but one thing I haven't
done much of is framing, and I've never used metal studs. I'm wondering
if I can use metal studs for the base and top sill - will they provide
enough support? Is there any reason NOT to use metal studs, and use
wood instead?

My other concern si not attaching the room to the existing structure. I
would like it to be a freestanding cube. Does anyone know if there is
any problem with this idea?

One last question - what is the best way to bolt it to the floor?