Essentially, yes. All of the surface of planet earth is ground level to us, whether a building exists there or not. You would then be on the first (man made) floor above the ground. Even a tent has a ground floor. Think of the ground as zero. Anything above counts upwards. Anything below downwards.

We do not use those descriptors in houses, like ever.

You would be downstairs on the ground, upstairs above that.

You might get specific and say "he's in the loft room".