u/Imaginary-Citron2082

I wanted to better understand the process after purchasing a premade ADU plan online. It is not a pre approved plan from the county yet. But I understand that I would have to get a title 24 as well as a structural engineer to change the plan so that it can pass the California Code. Can someone further explain this to me? And how to go about this? I'm very new at this, this is a project my boss is having me work on, and im having trouble to fully understand whats going on. 

What are the estimated costs of the process per each service? Do I have to go a certain order? Where is a good website I can look without them asking for my information and sending me a bunch of junk mail? Thank you.