I would even be willing to pay a small fee each month for them to hold on to my spares. 

like taking a loan for 110k for a house paid for 100k, they got 10k back at closing.its not like the money was free and clear, you have to pay that money back.i currently have 2 properties with a mortgage on each, every spare dollar i can find goes to paying down those mortgages.

In my spare time, I look for investment properties for myself.

@Jeff Caravalho, if you've got $80k to spare, why not go find some deals yourself?

Do you have the spare cash to provide 24/7 care for her for 75 years?

It's more of a side hustle idea for now, I still have a great W2 job in construction management.I already have a lot of my own forms and procedures, along with the 35 years of my dad's experience (including his garages full of spare doors and parts, tools, refrigerators, etc.).

A few of these units in spare will do the trick.

I used my spare time to study the many different ways to invest, from publicly-traded securities to direct real estate to private equity (angel investing).One of the opportunities I discovered was real estate investment trusts through Ralph Block's Investing in REITs book and the National Association of REITs site.

They are cheap enough that you could just by extras for spare parts.  

Its not the "house hack your way through to FI" or "anyone with a spare room in their grandmas basement can do STRs" So just beware its maybe not a great entry level place to start for most.