@Carl wrote:
If anybody lives in the property rent free for even one single night, it's a second home.
That depends. Per Section 280A(d):
(d) Use as residence
(1) In general For purposes of this section, a taxpayer uses a dwelling unit during the taxable year as a residence if he uses such unit (or portion thereof) for personal purposes for a number of days which exceeds the greater of—
(A) 14 days, or
(B) 10 percent of the number of days during such year for which such unit is rented at a fair rental.
