This is information the builder should have known before hand. Isn't that right?
No. No one will know that prior the actual piering. All piers are of different heights and is different for each site. Deepest was 2.8 m. few were 1.8m. They have to dig until they hit stone or clay or something solid where whole house slab can sit, else you will get cracks. Allworth Homes are pretty good in that. They allow owner to confirm the pier heights and confirm.
All of us has sewer line in front or rear, so nothing affecting for piering. The fact is most of Minto Estate is H1 soil and is a fill mostly. That is the reason they dig deep. House will be rock solid I am sure with that deep piering. 58 total piers.