AND you cannot separately sell or refinance that ADU in the future.That said, I ran across this company that seems to specialize in this type of addition: maybe they could help? 

Appreciate any guidance you can provide. 

So I'm in talks with a lender to get pre-approved and already have an agent who's been extremely helpful in providing spreadsheets of contacts for any specialists I'll need during the real estate investing process.

Also seek out local agents you specialize in STR. 

We wholesale nationwide and specialize in REO properties.

You haven't provided any details to go on.

Your question is very specific and I recommend seeking a CPA that specializes and actually invests their own assets in RE themselves to assist you.

Any suggestions, warnings, or references of service providers would be great!

At the end of the day, I do see a value in the general education side of many of these guru's, but education, if that is all they are providing without actually helping get your business off the ground, is only worth a flat fee, and it is not $540k, as you not this group is asking for.  

We specialize in30 year rental loans for investment properties that you can use for a cash out refi.