Some other threads in this section have pointed to invisible walls, barriers, ceilings, etc and frustrations from people trying to build extra levels or lofts in tall, thin houses: they buy a house hoping to build upwards, only to find it impossible. I wanted to start a thread so hopefully people could pool their information about houses and people can make intelligent purchases based on their plans. Maybe someone here will find it useful.
I gathered a few platforms and tried out building rudimentary second floors in some of my houses, then ran and jumped about on them to find bits that don't behave as they might appear. I don't have any the tallest races, so take my testing with a grain of salt if you do; I certainly can't claim to have been completely thorough.
If people find this worthwhile, I'll try to do a few other houses. Hopefully other people can share their experience with other houses, too.
Twin Arches
You could possibly get away with another level if you're really not claustrophobic. In the image, the max-height khajiit's head is just about touching the invisible ceiling, well below the apparent ceiling. To fit in larger characters, you'd be looking at a platform not much higher than a tall bookshelf. Probably not a great choice.
Captain Margaux's
Jumping around on various platforms here did not reveal any barriers where you wouldn't expect them. The posts and beams across the ceiling (from which the lamp hangs) do have collision and limit the scope of a make-do second storey, but otherwise this house does surprisingly well.
Hammerdeck
Or, where I store my junk. There's a significant divergence between the apparent and actual ceiling here; invisible barriers start around the first wood bit, just above the doorframe in the left side of the picture. It would be tough to fit in much of a loft here, even for short Bosmer.
Black Vine
I have actually built a permanent second floor here, as shown. Most of it works, but the wall on the left side of the picture, near the fireplace/chimney, has a lower invisible ceiling that extends a metre or so out - roughly to spot where the makeshift railing meets the angled grate/ramp. My character fits here, but can't jump properly, and a tall Nord/Orc/Snooty Elf might not fit at all without lowering the platform a bit. The other walls seemed to work like you'd expect.
Ravenhome
Extending the upper platform to make additional upper rooms works well, with collision pretty closely matching apparent architecture. The upstairs 'rooms' pictured in the bottom half are best; the room not shown (near the courtyard side) can be a bit fiddly at times and doesn't have a great path to getting in or out, but can still be used.
That's it for now. Hopefully this helps or at least does not irritate anyone. Thanks for reading.