First, you should note that you cannot
construct on top of constructed floors, but you can
build on top of them. There's a difference between these things, in that constructions are for making things like walls, floors, staircases, and ramps, while buildings are for every other type of building, including furniture and workshops. Constructions are a special subset of building that forms pseudo-terrain, and only other constructions are restricted from being built on top of constructions.
Second, if you do want to construct a structure with walls entirely out of constructions, bear in mind that walls can be built over open space. So, you just build the floors without the places you want walls, ramps, and staircases to go, then you build a single-tile bridge next to any places where there will be walls on an outside corner (so your dwarves can access those walls to build them), build those outside-corner walls, deconstruct those bridges, then build the rest of the walls and such.
Your problem, I suspect, is that you're missing the first point. You can build statue gardens on top of constructed floors easily since statues and related things are, in fact, not constructions. If that's not actually your problem, I suggest you look into the wiki's
article on Towers and other megaprojects, which more-or-less explains how to do such things. Note that while it refers to 40d, nothing has actually changed for 31.x, so it's all still totally valid.