The long term goal is to do custom homes, starting with spec builds.

My concern is that they added a room without permit(Not permitted work).

Make sure to check zoning laws and permit requirements in your area, and maybe connect with local contractors through referrals to get multiple quotes.

They offer flexibility in design and can be customized to include a business space.Shipping container homes: While trendy, these can be challenging for mixed-use (residential/commercial) purposes due to zoning and building code issues in many areas.

As soon as you start discussing your business practice and strategies with your customers (tenants), then all bets are off and you run the risk of Fair Housing violations. 

Also, I wanted the license to be able to pull my own building permits and do my own projects. 

First thing to do is check with the city or county to make sure the ADU would be permitted to be a STR.

Looking for some insight/ experiences from folks who have furnished rentals/ Airbnb in Anaheim (City requirement 31 day minimum unless you have a permit).  

Example would be for a SFH fixer upper in grandma condition:   $10k for demo and permits and prep, $10k for wall removing and layout revising, $10k for basement framing, $10k for drywall, $20k for paint, $30k for flooring, $20k for kitchen, $20k for baths, $20k for any MEP upgrades, $10k for trim work, $10k interior finishes 

After 18 months I still didn't have a building permit and decided to cut ties with the builder and forfeit my deposit.