A friend of mine lived in a hillside neighborhood. Wanted a small ADU (standalone unit, usually pre-fab, for office or in-laws). Reasonably priced prefabs. Since it was on an incline, couldn't just plop it on cement blocks. Got an estimate for a poured foundation. Slab was too heavy. It had to be the lighter kind that traced the exterior edge, with anchors into bedrock to comply with code.
$300K, just for the foundation.
Noped right out.