Can I write off interest payments on a personal loan from a relative, which is specifically for use in building my home?

[skimballsek1]

If I default on the loan, my relative gets the house and the land on which it is being built.

[SweetieJean]

Was a written loan agreement signed and filed with the county, securing the loan against the property?

[skimballsek1]

There is a written agreement, but not recorded with the county.

[Vanessa · Accepted solution]

Possibly.  You can deduct interest paid on a mortgage from a private party if all of the following apply:

• "Makes your ownership in a qualified home security for payment of the debt;
• Provides, in case of default, that your home could satisfy the debt; and
• Is recorded or is otherwise perfected under any state or local law that applies."

https://www.irs.gov/publications/p936#en_US_2018_publink1000229894

If your agreement with your relative meets the above criteria, then Yes, you can deduct the interest. 

[skimballsek1]

How do you perfect an agreement "perfected"?