Thanks to everything I’ve learned from the podcasts, books, and this community, my wife and I currently own:A triplex in MaineA single-family rental in San AntonioA single-family primary residence in Las Cruces, NM, which will become a rental when we move in a few monthsFor context, I'm an active-duty Army officer with about 12 years left until retirement, and my wife works for a university in Boston.

What is your primary reason for wanting to pay off the mortgage? 

Even in HCOL areas, adding an ADU typically does not make sense because developing a single small unit is the most expensive residential development that exists.Adding an ADU is a lot of work (probably more than a brrrr but with worse return), has poor financing options, typically adds less value than the cost of the addition, and detracts something from the primary structure even if it is just yard and privacy.Good luck 

From tax stand point I would think you can separate from primary and be okay with IRS being that IRS will not be local code enforcement.  

I live in my own primary residence and that is fully paid off. 

However, consider STR seasonality, local regulations, and management demands, and ensure your equity loan and new primary residence costs fit comfortably within your budget.

Buying a property as primary, renting a bedroom or two to friends is a good way to start building your RE portfolio.

Your primary insurance policy with $300,000 in liability coverage should be sufficient in 99.999% of all lawsuits.5.

Features a versatile layout with potential for office or hobby spaces, and a primary suite with a double vanity bath.

Is this all handled by the PMS and then routed to the guest by whoever is the primary host?