Technically, any residence can be a home – but not according to the IRS. In order to qualify as a home for tax deductions, your home must:
Serve as collateral for the loan (in other words, what's at stake if you do not pay back the loan)
Be a house, condo, apartment, co-op, mobile home, houseboat, or house trailer
Have separate sleeping, toilet, and cooking facilities
Your home can be one that you get in order to "buy out" an ex-partner's half of the house in a divorce. If you get a nontaxable housing allowance from military or ministry service, that can also serve as a qualified house.
For a second mortgage(or rental property) you wish to claim tax deductions for, the above situations apply. The home does not have to be used for the entire year. However, you have to be there for whichever is longer – 14 days or 10% of the number of days you rented it out.