Rich
Span the 10' distance with a 2 x 10, or preferably, a 2 x 12. I would double the outside joist. You might want to put a supporting post (Like a 4 x 4) at each end of the doubled joist.
You can certainly use the exterior walll of your dwelling. When you attach to this, hit the studs. (Luke, use the stud finder!)
Be sure to allow room on top of the joists for flooring so that it will be level with the existing loft floor.
If you need to use a thinner joist, then you will need a load bearing wall or an I-beam with several posts to support it.
I would say that there is more than one way to skin a cat, but I think that PETA is watching. So, know there there is more than one solution to your architectural conundrum.
JayR
