As in the common rent for properties of the same size that you can put a box around on a map. 

You are suffering from a common misunderstanding of the law.

The most commonly listed minimum is 5.5sqft.6 - Heating and Cooling element.

Illegal basement units seem to be very common in this area.

He paid for common area access, built a bed in his Land Cruiser, pocketed the rent but we had agreement on what the pocketed money could be spent on (seeing we were paying the rent). 

Pretty common for the area to do work like this

I highlighted two common exceptions in my earlier post, but doesn't sound like either scenario applies to your case. 

Being the experienced PM that you are and professionally screen your tenants on your future rbr HH, I think that you will be able to debunk (not sure if that is a word) all of the common rbr misconceptions.When Acorn decides to take on rbr pm, lets connect!

https://www.biggerpockets.com/...12 Frequently Asked Questions (And Answers) About DSCR Loanshttps://www.biggerpockets.com/...8 More Commonly Asked Questions and Answers to DSCR Loanshttps://www.biggerpockets.com/blog/eight-questions-and-answe...What Documents Do You Need for a DSCR Loan?

When converting common living areas into additional bedrooms, many DSCR lenders might view the property as non-conforming, which could pose challenges.