The IRS places several limits on the amount of interest that you can deduct each year. If you took out the mortgage before Oct. 14, 1987, all of the interest on the mortgage can be deducted regardless of the amount you borrow. For all other mortgages, you can only deduct the interest that accrues on the first $1 million of mortgage loans. The average mortgage in the United States is around $185,000 so this deduction covers virtually of all Americans.
If you are married and file a separate return from your spouse, then the limitation decreases to $500,000. These limits are cumulative for all of your mortgage debt on both homes. For example, if you are single and have a mortgage on your main home for $800,000, plus a mortgage on your summer home for $400,000, you would only be able to deduct the interest on the first $1 million, even though both loans are each under the $1,000,000 limit.