figuring out foundation needs for hillside home

I have family members who own a home in the Berkeley hills on a particularly steep lot.  It has potentially serious foundation issues.  I'm looking for pretty much all advice about how to figure out what might be required and what it might cost to address. The house is 2 stories + basement and probably 1000 square feet on each floor.  It's about 100 years old. In particular...

* Do you have ballpark thoughts about what a new foundation might cost in this situation?  Did you have major foundation issues and fixed it up and are willing to share your approximate costs?

* If you had foundation issues, what was the process of identifying requirements and getting quotes?  Do you need to hire a structural engineer first for a full report, or can foundation companies do quotes directly?  (Please use as much detail as you're willing to share!  Use small words!  Even smaller!  Pretend that I am a moderately competent person who knows basically nothing about foundations but am trying to help family members who know even less.  Wait that's exactly the situation.) I'm trying to help them come up with a process that will get them the details they need.  

* If they decide to sell instead, are there concerns about having structural engineer reports to disclose?  It sort of seems to me like anyone who looks at this house will think "huh that's steep and old, what about the foundation", but maybe I'm wrong?

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I live in the flats and can tell you that our foundation was redone before we bought our house and the sellers spent $60k on it. The inspector was able to see it needed work. The sellers originally listed the house but the listing fell through and then they had no other takers so they decided to fix the foundation before re-listing. When we bought from them, we kept $50k in escrow until we got sign off from the city that the work was done properly, which it was. They used Ned Clyde Construction.  

You need to hire a geotechnical engineer (civil engineer licensed in California, specializing in soils and geology) who can figure out what is causing the foundation issues.  They will do a subsurface investigation of the soil/ rock and provide design recommendations to your structural engineer (separate licensed civil engineer specializing in structural design) who will perform structural calculations and provide design drawings for foundation repair.

We owned a 100+ year old home on a not too steep hillside with foundation issues from the Napa earthquake until last year. We didn't get as far as fixing the foundation (We began looking for contractors right when the pandemic hit and the quotes had doubled between when we bought the house and when we thought we could afford to get the work done) but this is the process we did: 

Structural engineer inspections: Level study and foundation inspection about $1200 total
Soils test and report: $2-2500
Foundation replacement plans from structural engineer: $10,000
(all of these must be done to get the project permitted by the city. None of our plans needed revisions.)

Quotes for the work ranged between $120K to $180K in 2020. 

We sold the house last year before fixing the foundation, but we disclosed all the preliminary reports and plans we had to buyers. We made a decent profit on the house, even with the foundation issues, and found a buyer before the house was even on the market. It might be harder to sell today than it was last year, but the market is still so tight in the Bay Area, you'd be surprised what people will be willing to spend.