For my younger daughter's birthday, I built her a new lofted bed. Easy enough in scope, but I wanted something that could be taken apart, maximize headroom both above and below, and not cost an arm and a leg.
4x4 posts with 2x8s around the perimeter. Those are cross-bolted both ways. The long sides are also inset into the 4x4s for added strength. Under the bed, there are 2x4s for support and rigidity. Those are inset into the 2x8s to maximize space. Her dresser needs to fit underneath, her box spring and mattress must fit up top along with her being able to sit upright on top.
It's totally solid, assembles quickly, and she loves it. Unfortunately, my sander died and I had to finish sanding the ladder and top rails by hand. I will be hanging a string of Christmas lights underneath for light and she's going to get a curtain rod and some sheer curtains to block off the underside.
Actually, the dorm one I built when I was in college was way easier. I took 3 4x4s that were almost ceiling height, put carriage bolts in the top end with double nuts. Use a 6x6" piece of scrap oak on top of those. Unscrew carriage bolts to tighten the 4x4 up against the oak which then presses against the ceiling. Lag screw frame to the 3 4x4s. The 4th corner will be the one in the centermost part of the room. Use lag screw to hold chain to 4x4, and bolt other end to frame. Another version is to use chain on 4th corner from both closest 4x4s.
Most of the time in college, I slept on my couch underneath it anyway.
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Bryan-NW 'burbs 1972 Malibu Vaguely stock appearing, and the opposite of restored. 1999 std bore 5.7, Vortec heads, Holley Stealth Ram, GM cam 700R4, Viking coilovers, 12 bolt 4.10 posi, and a whole bunch more