For you to take a home mortgage interest deduction, your debt must be secured by a qualified home. This means your main home or your second home. A home includes a house, condominium, cooperative, mobile home, house trailer, boat, or similar property that has sleeping, cooking, and toilet facilities.   

(Note: If you have a second home and rent it out part of the year, you also must use it as a home during the year for it to be a qualified home. You must use this home more than 14 days or more than 10% of the number of days during the year that the home is rented at a fair rental, whichever is longer. If you don't use the home long enough, it is considered rental property and not a second home.)

The interest you pay on a mortgage on a home other than your main or second home may be deductible if the proceeds of the loan were used for business, investment, or other deductible purposes. Otherwise, it is considered personal interest and is not deductible. 

Under the new tax law, effective in 2018, the deduction for interest paid on home equity loans and lines of credit, is suspended unless they are used to buy, build or substantially improve the taxpayer’s home that secures the loan.

Also, beginning in 2018, taxpayers may only deduct interest on $750,000 of qualified residence loans. The limit is $375,000 for a married taxpayer filing a separate return. These are down from the prior limits of $1 million, or $500,000 for a married taxpayer filing a separate return.  The limits apply to the combined amount of loans used to buy, build or substantially improve the taxpayer’s main home and second home. 

>>See this for more detail:  https://turbotax.intuit.com/tax-tips/home-ownership/deducting-mortgage-interest-faqs/L4a9KF9mI