Unfortunately, the builder was over their head and went bankrupt.

You and the other posters here have been generous in indulging me in this exercise of considering this "partnership" / BRRRR, but demonstrating that we can invest successfully on our own power before seeking out partners is not only my strong preference but just sound business logic.As a new investor my head starts to spin when I start to think about investing, and the many gaps in my knowledge reveal themselves: how to structure a partnership? 

Either you don't know why because you have your head in the sand and haven't paid attention to their communications or they are trying to hide things from you. 

If we bite off people’s heads when they admit disaster, it suppresses legitimate issues with these techniques.Just for context there is a lot of history with the OP and some of us on here going back years (like the thread I linked to above from about a year ago and several others if you use the search function with his name) with him minimizing the risks of how he was structuring these transactions and some of us being cautionary, warning him that exactly what happened here could happen, which made him defensive and dismissive. 

So you might as well be a head of the game do the cash out refinance buy the next 1-2 properties and simply refinance the original mortgage as a "rate and term" which carries a lower rate.

"Head right on back to the back of the room and my associates will get you signed up today!"   

That in itself is a skill and a needed over head component to keep from having investors get very antsy and then take to SM because they are not getting their e mails returned that day or next .. 

Also take a chance and look in 'D' areas that you suspect that are heading upward (gentrifying) towards C or maybe even B.

Being a normal person and giving someone a heads up has not bit me, yet.In addition to what the law requires, I always shoot the tenant a heads up either via text, call, or email.If they are mostly responding to email, then shoot them an email.Emojis and gifs work wonders in today's day in age.

He added that Mr.