Looks like parastitic plants from lumberjacking, gnarled staves, crafted and looted since reversed. Some nightshade, and grapes? lol Tribal berries and oragami butterflies too. lol
You could do it two ways, easiest way i'd say is to build a support walls on the first floor, and add flooring on the second, go to customize and delete the floor and fix the supports to what you want visable.
Second way, You could build a goza platform with stairs and do it that way and just chop after you build it.