If you really do need a one-word-fits-all translation for terrasse, i.e. you can't say 'patios and balconies', this would be a way of getting round the issue.

I'm also assuming that the named technique, sur plots, is 'incidental' and that the issue could arise with raised flooring placed on old bricks and slate shims, for instance, as has been done.

However, I suspect the real issue is not the raised flooring itself but, since we are talking about étanchéité, about the underlying structure. Raised flooring, be it slabs or strips, will necessarily not be waterproof, i.e. the slabs or strips are simply placed on their supports, without any sealing, so rain, as with spilt wine and beer and toddlers' (or drunken men's) wee will fall straight through to the underlying surface. And while the people on the floor below your balcony might not object overmuch to rain penetrating to their otherwise protected-by-your-balcony balcony, I suspect beer, wine and wee might be a bit much for them.

With raised flooring at pretty much ground level, there could be an étanchéité issue if water pools beneath the flooring and seeps into the ground, at least if the seepage point is close to the house (the point of patios is often to keep water away from the foundations).

So, reading between the lines, I'd say something like waterproofing of surfaces beneath raised outdoor flooring. I do understand that that will create hell for your machine-translation program.