To make it more complicated every county has different valuation processes and different equations to figure the taxes. 

It is more complicated, but it gives you more power and less risk.

I do not want to attract unnecessary audits, IRS problems, or any other. 

Granted that would be a year from now but doesn't hurt to be aware of potential complications in the future

The firms focus is in commercial property and I have learned an incredible amount about navigating city and county regulations, dealing with complicated leases, permit running and expediting and so much more.

I wouldn't be putting any money into the deal, just the time and know-how (such as it is).Complicating the issue is he has a brother who is potentially interested in kicking in money as well to make a deal work.So how do I structure this?

For your example we'll need to make it more complicated an make some assumptions along the way.Scenario 1 Assumptions1 - You pay out of pocket the entire cost to build your ADU, cash invested $100,0002 - The value of the property does not justify the fees associated with cost segregation so you will use a 27.5 year  depreciation schedule.

If I add an ADU in the back, having five or six cars would be a bit more complicated..

You'll want to make sure it's fair market rent, or else things get even more complicated. 

I don’t like the idea of causing unnecessary bad blood between family by force selling the property and having it divided up.