Loft bed for teen

Maybe not quite what you're looking for, but we got an excellent loft bed from College Bed Lofts, https://www.collegebedlofts.com/.  They sell a kit of cut and sanded pine with all the bolts and hardware, as well as detailed instructions. If you have a drill or power screwdriver and maybe a helper, you can put it together really easily. Or you can hire a handy person.  Either way, it will cost less than building from scratch. It's a very solid bed, no particle-board. They have a lot of customization options as well, for adding a desk, shelving, etc. We did our own customization by getting a double-bed sized loft, and adding a couple of boards along one side of our daughter's twin mattress so she has a flat space, like a little floor, next to her bed. We also painted it, though that's not necessary.

Your post calls for a loft with walkable space underneath.  This will require a high ceiling, on the order of 11-12 feet.  Figure that the mattress and its support will be at least one foot tall, and that the sleeper will need a few feet above the mattress to be able to sit up.  Our daughter is pretty short, and she has 3'8" from the mattress top to the ceiling -- we wouldn't want less.  Our ceilings are at 8'4".  Subtracting the mattress, slats, and above-bed space leaves her 3'8" beneath the bed.  She has a rug and a bean-bag chair, kind of a private cave under there.  She really likes it, but it's not walkable space.

A final note: heat collects at the ceiling, and with the bed in the corner there isn't much air circulation.  It can get pretty warm up on top of her bed unless we run a fan.