After that one meeting the engineered plan was recorded at the courthouse and we could sell individual units. 

My solar system was grandfathered into NEM 2.0 with Pacific Gas & Electric (PG&E). 

A company comes out twice a year and inspects the AC and replaces the filter (very important if in a high temperature area), inspects the plumbing and electrical, and provides feedback regarding any potential issues.

Depending on the lot if it is in the county's critical slope designation the foundation is required to be engineered.

What I had in mind was having a builder handle foundation, framing, roofing,electrical, plumbing, etc.

The most pressing, and i cant seem to find a true answer from the the building department OR the utility companies is if I need to have separate meters (Electrical, gas, water) for my ADU and main home. 

They can do pretty much any type of inspection and have engineers on staff.

It has electrical, but no plumbing.

@Kevin McKittrick Well in my opinion an electrical upgrade should be paid for by the owner of the property.

It turned out there were aluminum wires so we upgraded the electrical service, removed all of the walls so we could rewire the house (4 bed 2 bath).