ya except the builder isn't going to go sell him the land unless he agrees to build the new house for him

lol I know it's actually not hollow. To be honest I'm not entirely sure what happened. Maybe the soil below has dropped or some how it didn't settle below the rebar, but every time you walk on the floor, you can hear a distinct echo below your feet. Has a very metallic sound to it. Regardless it's annoying as crap and is something I've never heard before on a foundation.

It's likely a void beneath the slab, which typically occurs when they dig too deep during excavation and back fill to intended level. One solution is to inject foam under the slab to eliminate the empty space.

you always over excavate...but it's normally only by about 5-6" to put down a layer of 3/4" rock (and poly) prior to pouring the slab

but as you say, if it was over excavated by a decent amount then back filled and not packed it theoretically could have settle but generally the slab would have settled a little with it (slab on grades are generally not reinforced or just have wire mesh) and you'd see a lot of cracks

but anything is possible

THIS.

New regulation from the city is so fucked. The shit they make you do to make it more "green".

Which custom designer did you end up going with?
I'm looking for referrals to get a custom home design done.

This is what most likely happened. Instead of spending so much time compacting the backfill they'll just fill it in and it will settle over time.