I bought a house in 2020 using a 1031 exchange. My inlaws pay at about 1/2 going rate. Don't make a profit. Should I declare the it a rental or wait I rent a fair rate?

[Critter-3]

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[Mike9241]

days rented to family members are personal-use days unless fair rent is charged and the unit is the family member's main home.  

was personal use greater than 14 days or 10% of days rented

if yes and if rented for less than 15 days you must treat it as a personal residence, do not report rental income or expenses. if it is a qualified residence  (1st or 2nd home and mortgage totals don't exceed the maximum allowed) the interest and Schedule A deduction. Real estate taxes also are reported on schedule a (there's a $10,000 maximum deduction for taxes on Schedule A) 

if no, prorate expenses between personal and rental use. personal portion of mortgage interest (also subject to the maximum limit for mortgages on personal use homes) and taxes on Schedule a,  rental portion of interest and taxes are not limited to rental income. other deductions are limited to the remaining net rental income and are subject to ordering rules. carryovers are allowed. 

whether rental to a family member would jeopardize the 1031 deferral is a question that would need to be answered by a tax pro in your area.