Did the structural plans change?

Simply ask why additional expense is required at this time.

My guess is that 10% down payment for a simple garage is not comparable to 10% for a habitable dwelling. You have made a substantial change in the scope and budget for your project. If you had started the project as an ADU, your 10% initial payment would have been substantially greater, don't you agree?

If I was the builder I would be wondering about your integrity: starting cheaply and then changing your mind, after the initial payment, to a much more expensive project which he will have much greater start-up expenses. On the surface, it could be interpreted that you were just looking for a strategy to reduce your initial payment to the builder. Put yourself in her/his shoes--what would it look like to you?

All of that said, I don't know why the work/fees for soils engineer and surveyor would change, unless the coverage and configuration of the design has changed substantially from a garage to the habitable dwelling, and if foundation locations and types have changed. Have they?

Ask for an explanation of this aspect of the work.

Since you have substantially changed the scope of the project, you should expect additional work on soils, slopes, land survey-even permits- in your new concept.

Get a newly designed contract, within your new build. It may be radically different. Not the contractor's choice or fault.

Is the contractor not answering these questions? He should be able to explain it better than any of us, and you have a right to ask.

A "soil test" or geotechnical analysis is potentially a complex report, which may answer a question like, "Given the requirements of this structural design, what is the ability of the site to accommodate the structure? What are the issues in detail?" So, even within the same building foot print, if the structure changed as to weight, anchoring methods, foundation, drainage needs, etc. the analysis may need to be redone. OTOH, since you're paying for these tests and reports, you should know exactly what was in the first one and in what way, if any, the first report fails to cover the contemplated changes.

https://en.wikipedia.org/wiki/Geotechnical_investigation

Once you changed the use of the land everything changed .

Sorry to break it to you but the contractor is correct here. The moment you change the structural backbone of a structure the soils engineer needs to get involved and write a letter to say the intended use of the structure is supported by prevailing soil conditions. It is probably fine for all you know BUT it does need to vetted and anytime any engineer puts pen on paper saying something is structurally correct that usually is a liability which ends up costing money to the homeowner.

By the way, I am also in California, just north of you. There are many instances when i felt like I had to pull my hair out when something like this happened in my new build. All said and done, you are protecting yourself by having these engineers do analysis for you. Would you be okay if there was a landslide or if the subsurface water wasn't discharged correctly and that sunk the foundation or whatever..?

Let me ask it this way, do you know if the structural calcs changed because or the change order? Do you know that the structural calcs are based off of the soils report and it is the soils engineer who makes certain recommendations and the structural engineer looks at that and derives load bearing calcs from that, based on soil conditions.

Maybe the change order needs the foundation or stem walls to be changed or the piers for retaining wall needs to be changed or maybe them need to be deeper or maybe the forms need to be #6 instead of #4. All i am saying is the soils engineer cost is warranted and you should accept it as part of being a change order.

Norman, you seem to be willing to "move" within all that it takes to get the new concept. That takes guts, and obviously you see opportunity. Good for you!

I renew my call to work with your contractor/GC on a new contract for this new concept. Maybe it's just on one part of it all, but protect yourself against the brain damage and expense of change orders. Nothing better than clarity to make it all go smoothly, and allow you and your contractor to be better partners.