Home inspection is a good start of any construction project. This might be a surprising but true turn of phrase. This is true for your accessory dwelling unit construction too.
Things to watch out for: old roof if you plan to install solar panels for your ADU on the main house; septic tank as the city might ask you to get hooked up to the public sewerage line; size of your black water pipes, as a 3 inch diameter pipe for the main lateral is not enough to handle additional volume; old distribution panel, as it might not have enough reserve for all the appliances in your new ADU.
When one knows that the average home in California had been built before 1970, it is not surprising that a lot of so called hidden costs or change orders stem from failing on due diligence before the project ( the industry term is pre-construction phase).
A good solution might seem to have your contractor walk the job before you start. But what about the conflict of interest? After all, his compensation comes from the number of line items in your contract.
Might be safer to get a third party opinion. Home Inspector. The question is how to select a good one.
Here is a link to a list of questions you might want to ask your potential home inspector – coming from a home inspector in a different state (to avoid conflict of interest of course:): check out Inspectagator.
What if the inspector finds anything that needs to be fixed? Get minimum three bids, of course. And if you are planning an ADU project, discuss with your ADU builder if it makes sense to wait or get it fixed fast.