Using planks for floors/ramps doesn't take up much room because they are big. I've used them quite a bit in my home along with orcish bars for the edges. The planks are rough which makes it almost impossible to put rugs down though.
Here's a screenshot of a floor I've built using planks:
You can use the "bolted" style paintings turned upside down as a floor, or the daedric platforms work well too (the purple diagram one, I think it's called daedric base, ashen). The orc platforms work well but are kinda thick, which is why the thinner daedric bases or bolted paintings might work better. Good luck!!
PS--this isn't the same house, but it's an example of using the daedric bases as a floor and stones for the stairs.
Lasertooth
GM of ESO Grand Designs, Grand Designs Too, and Grand Designs Trinity
Xbox/NA