You want a house on the hill? Kick him off of it. Or add 100K to your cost.

I'm afraid it's too late for any kicking off, we've already sunk money into site prep and concrete.

Typically house is built on undisturbed soil, you have to have some good bearing capacity to hold the house... There are alternatives like pier and beam foundations [massive pilings you mentioned] , massive compacting, etc. Now compacting 20' of soil - is a different level of crazy.

My engineering class professor was telling us how a lot of roads today build by compacting, but it is only done few inches at a time. Purchasing, getting it delivered and compacted 20' of soil?

What wild expense and feats of engineering would this take to accomplish?

Why ask on this forum... Depends how far you need to go and how many dump trucks you need, how much compacting you need. And the prices are always local - 100,000 ... 300,000; California $1,000,000 who would know.

Are the basement footings on top of the fill or are they on "undisturbed" soil? That would be ideal. If the footings have to be on fill, I agree that a soils engineer should be consulted. An engineer will determine how much topsoil to remove, what type of fill should be brought in, and how to place the fill. They may ask for testing of the fill in place to make sure it has been compacted to spec.

My house was built on fill. They performed 8" lifts (at a time), compacted with the heavy equipment and then the soil engineer came out and took density and moisture ratings using very specialized equipment (somehow radioactive, and required notification to the state of his whereabouts all the time). There are requirements for the soil to be xx dense at less than xx moisture ratings. That meant there were times that the grading folks had to till it up after compacting to speed drying and then compact.

We had an additional complication in that we were constrained inside of retaining walls. If you can slope it down gradually it will be easier.

In some areas, we raised 11'. We know precisely the content of the soil and that there are no voids or underground streams, etc.

It absolutely can be done and it's not terrible. And - when we dug footings - we had no "surprises" which can be more costly than the fill.

My advice - get good graders and do the lifts and the testing. My house walls and slab have been in for 2 years (and yet I haven't moved in yet - long story) but there has been ZERO settling.

The fill cannot contain any organic material so rather than dirt it will be graded granular material.

Perhaps someone else touched on the idea that of constructing footing on undisturbed soil and adding piers to the desired level of the structure then filling around them.

Could the workshop be moved to a new foundation on the lower part of the site and its old foundation modified for a new structure?