The Mortgage Interest Deduction: What the Rules Actually Say

What the deduction is, structurally

Federal tax law lets taxpayers who itemize deduct interest paid on certain home loans. The controlling reference is IRS Publication 936, which defines every term that matters: what counts as a qualified home, what counts as deductible mortgage interest, and how much debt the deduction can apply to. This page explains how those rules are structured so Publication 936 is easier to read — it does not tell you what to deduct. That is a question for the publication itself and, for anything non-obvious, a tax professional.

Itemizing is the gate

The deduction only exists for taxpayers who itemize deductions instead of taking the standard deduction. Many homeowners' total itemizable deductions do not exceed the standard deduction, in which case the mortgage interest deduction — whatever its theoretical size — changes nothing on their return. Any back-of-the-envelope math about "the tax savings of buying" that skips this step is wrong before it starts.

"Qualified home" is a defined term

The interest must be on debt secured by a qualified home, which Publication 936 defines to include a main home and one second home, with specific requirements — the loan must be secured by the home, and the property must have basic living facilities. A property that fails the definition produces no deductible mortgage interest regardless of how much interest was paid on it.

The debt limits, and why the loan's date matters

The deduction applies only up to a ceiling of mortgage debt, and the ceiling depends on when the debt was taken out. Under Publication 936, for home acquisition debt taken out after December 15, 2017, interest is generally deductible on up to $750,000 of that debt ($375,000 if married filing separately); older debt is grandfathered under a higher limit. Two practical readings follow:

• If your mortgage balance is under the applicable limit, the limit machinery mostly doesn't touch you.
• If you carry large or multiple home loans, or refinanced older debt, the date-based rules interact in ways the publication spells out in detail — this is precisely the territory where a tax professional earns their fee.

Note also that the rules distinguish acquisition debt (used to buy, build, or substantially improve the home) from other borrowing secured by the home. What the borrowed money was used for affects deductibility, not just what secures the loan.

Form 1098: where your number comes from

Each January, your loan servicer sends Form 1098, reporting the mortgage interest you paid during the year. This is the figure tax software and preparers start from. Points paid at closing have their own treatment — sometimes deductible in the year paid, sometimes spread over the loan's life — and Publication 936 covers the tests that decide which. (For what points are in the first place, see how to read a mortgage rate.)

How to use Publication 936 without drowning

The publication is long because it handles every edge case. A practical reading order: start with its definition of a qualified home, check which debt-limit category your loan falls into by its origination date, then read the section matching your situation — refinancing, points, or multiple homes. Bring the specific questions it raises to a tax professional rather than resolving ambiguities by guesswork. Tax rules change; the publication is updated for each tax year, which is exactly why this page describes its structure rather than freezing its details.