To enter mortgage interest:
Per IRS Publication 936 Home Mortgage Interest Deduction, page 2:
Note. Interest on home equity loans and lines of credit are deductible only if the borrowed funds are used to buy, build, or substantially improve the taxpayer’s home that secures the loan. As under prior law, the loan must be secured by the taxpayer’s main home or second home (qualified residence), not exceed the cost of the home, and meet other requirements.
Fully deductible interest. In most cases, you can deduct all of your home mortgage interest. How much you can deduct depends on the date of the mortgage, the amount of the mortgage, and how you use the mortgage proceeds.
If all of your mortgages fit into one or more of the following three categories at all times during the year, you can deduct all of the interest on those mortgages. (If any one mortgage fits into more than one category, add the debt that fits in each category to your other debt in the same category.) If one or more of your mortgages doesn’t fit into any of these categories, use Part II of this publication to figure the amount of interest you can deduct.
The three categories are as follows.
The dollar limits for the second and third categories apply to the combined mortgages on your main home and second home.
See Part II for more detailed definitions of grandfathered debt and home acquisition debt.
You can use Figure A to check whether your home mortgage interest is fully deductible.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.