I would remove the old structure completely.

The pool has been buried for 25+ years and filled with soil. So why would removal be necessary? To my knowledge, a 10x20 plunge pool would not need nearly require as much space as would be given within the old steel frame.

Yes, the new pool would fit nicely but, that old structure has the potential to be a problem for any future digging or work around it. You will have run plumbing and electric (lights) to the new pool. Now your faced with finding a path through the old structure to do that.

I'll bet the old concrete pool patio is buried in that hole when the pool was filled in. Maybe some other "surprises" down there.

To me, it's buried construction debris and it should be removed and disposed of properly.

Appreciate the explanation- we have done an exploration dig and only soil is buried underground. Sigh, we may have to settle for a swimspa after all. Weve been quoted around 20k for proper removal, and I just cant justify that.

Being steel, the entire structure would need to be verified as continuous and included in the bonding grid to be safe as a surround for a new pool. Conductive materials within 3 feet of any pool including underground can become energized and transmit deadly voltage. You can die from a shock of just 6 stray volts stepping out onto wet grass or a patio that has unbonded, conductive material underneath. To be honest, setting a swim spa over this same area could be less than safe for the same reason. Swim spas are often not installed with the most ideal safety considerations when it comes to equipotential bonding. Consult an electricial who specializes in pool bonding to verify. If you can safely annul the threat of underground steel, San Juan Pools makes a couple of small pool fiberglass models that can be installed completely above ground, the Sedona, Canyon Lake, and Canyon Lake Deep. They will ship you the shell only if you cannot find a local dealer of San Juan. Alternately, I would not install an in-ground fiberglass pool prior to removal of the steel shell because you will not have room for proper plumbing and backfill, plus the ground may not be secure enough to prevent shifting under lateral pressure of the new inground installation.

Hi there I'm also in Ontario and have a similar situation. Just wondering what you ended up doing.