On Mar 4, 5:59*pm, Molly Brown wrote:
I saw what looked like a septic tank cover right next to this very
expensive house built on a slab foundation. The house is on a hill and
the ground falls off right after where this cover is so the only place
the septic tank could possibly be, I would assume, is under the house.
Is this possible?

No. Tanks are not allowed under the house foundation.

However I have seen it done on older construction. Much older.