Or you can sell it, split the equity, and go separate ways.I'm not a fan of borrowing from family/friends unless all parties are honest and everything is documented in a written contract.

If you keep borrowing against the property you can immediately use the money for other investments.

If you want to buy another rental, then saving your money for that (assuming you have room to borrow more money) is probably the better idea.

@Michael Challenger I scraped all the cash I could find and borrowed from family.

That may not apply to you, as a financial professional, but I thought I would share the story.Obiously you could re-lever your portfolio to bring the ROE back up, but then you have to place those borrowed funds somewhere that nets a high enough return that the portfolio is ahead, given the drag of the interest and any transaction fees.    

Totally agree about the "bigger bird, smaller stone" analogy (can I borrow this?)

Commercial properties can be lucrative, but the financing/underwriting required from lenders is generally more "conservative" relative to residential and usually have higher interest rates (unless you have a "rock star", long-term commercial tenant in place) and generally require more equity from the borrower compared to residential.

Anyone who managed to do this probably did it 5+ years ago when the market was very, very different.If you borrow $200,000 to build an ADU, I suspect you would need to rent it for $5,000 or more to make the numbers work.

The source of the loan does not matter.So yes, if you borrowed $100k and used $50k to buy B and another $50k to buy C - you deduct half of the interest against B and the other half of the interest against C. 

You'll probably need a co-borrower and someone to gift the down payment.There's a Conventional loan (FNMA family opportunity loan) that allows your parents to purchase a home for you (and potentially roommates) to rent while you're still in college.