This is a solid mulit level house that is ready to move in and ready for your improvements. Roof seems good, older Anderson Windows, large lot (ingrained pool needs to be filled in ).True, an eggcorn is typically inspired by a word that's somewhat opaque, and it's hard to imagine a more transparent term than "in-ground pool." Nor is in-ground a recent coinage. The OED dates it to 1973 -- "orig. U.S., of an outdoor swimming-pool: built into the ground (as distinct from one placed above ground), esp. at a private residence." And Google News turns up a 1962 ad in the Milwaukee Journal, seeking franchisees to sell a "low priced inground pool to reach mass market."
So how does in-ground become ingrained? I think the connection must be the (relative) permanence of the hole-in-the-ground pool; ingrained originally meant "dyed," and it still means "deep-seated, worked deeply into the texture or fiber" (AHD, via Wordnik). An above-ground pool is removable; not so the ingrained kind.
If I were one of those word watchers who can read minds, I suppose I would accuse the "ingrained pool" people -- Google turns up a couple dozen of them -- of "trying to sound elegant," or something like that. Alas, I seem to be missing the telepathy gene; all I can do is record this interesting substitution.