Issues

Mortgage Interest Denied? Why the IRS Cut It Back

The IRS slashed your mortgage-interest (or investment-interest) deduction. The deduction is real—but it's hedged with traps. Here's how to win it back.

You deducted your mortgage interest—real money you paid your lender every month—and the IRS cut it or wiped it out. Or you deducted the interest on a margin loan you used to buy stock, and the IRS capped it at a fraction of what you claimed. Either way, a notice now says the deduction you thought was automatic isn't.

Here's the reassuring part first. These deductions are real, and interest disputes are among the most document-driven fights you can have with the IRS—the answer is almost always sitting in a recorded mortgage, a Form 1098, and a paper trail of where the money actually went. But the deductions are hedged with traps that catch people who did nothing wrong morally and everything wrong technically: whether the loan was secured by your home, whether you stayed under the debt caps, what the loan was actually used for, and whether you're even the person entitled to deduct it.

This is the substantive guide to winning that fight. It covers two different deductions that share one backbone—home-mortgage interest and investment interest—because both are the same move: climbing out of a rule that says your interest isn't deductible at all. It pairs with How To Respond to a CP2000 Notice, which owns the step-by-step response procedure, and How To Get and Read Your IRS Transcripts, which owns pulling the data the IRS used. This article owns the interest deduction itself.

The One Rule Behind Both Fights

Start with the rule that controls everything: for an individual, interest is presumptively not deductible.

IRC § 163(a) opens generously—"there shall be allowed as a deduction all interest paid or accrued within the taxable year on indebtedness." Then § 163(h)(1) slams the door for individuals: "no deduction shall be allowed... for personal interest." Credit-card interest, personal-loan interest, late-payment interest on your taxes—all nondeductible, full stop.

So everything that follows is you climbing out of that knockout through one of a short list of exceptions in § 163(h)(2). This article covers exactly two of them:

Qualified residence interest—your home-mortgage interest, under § 163(h)(3).
Investment interest—interest on debt used to buy investment property, under § 163(d).

Both share the same shape: your interest is presumptively nondeductible personal interest unless it fits a specific exception—and even then, only up to a limit. That shared structure is why one article can cover both.

Two close cousins are out of scope here, so don't confuse them. Student-loan interest is a separate deduction under IRC § 221—an above-the-line deduction (up to $2,500, with an income phaseout), not the itemized deduction this article is about. And the brand-new car-loan interest deduction (a temporary 2025–2028 break OBBBA added) is its own animal with its own caps and rules. Both deserve their own guides; neither is what the IRS is fighting you about here.

Home-Mortgage Interest: § 163(h)(3)

To deduct home-mortgage interest, your loan has to be "acquisition indebtedness"—and that word carries three requirements packed into § 163(h)(3)(B). The debt must be:

used to buy, build, or substantially improve a qualified residence,
secured by that residence, and
within the dollar cap.

Miss any one and the interest falls back into the nondeductible personal-interest bucket. Let's take the pieces that actually decide disputes.

The Debt Caps—and a Big 2026 Change

There are two caps, and which one applies turns on when you took on the debt:

$750,000 ($375,000 if married filing separately) for debt incurred after December 15, 2017.
$1,000,000 ($500,000 MFS), grandfathered, for debt incurred on or before December 15, 2017 (and certain refinancings of that older debt, up to the old balance).

Here's the currency hook that matters for 2026 returns. Under the 2017 tax law, that $750,000 cap was written to expire after 2025 and snap back up to $1 million. A lot of older guidance still assumes that reversion. It didn't happen. The One Big Beautiful Bill Act (OBBBA, P.L. 119-21, § 70108), signed July 4, 2025, made the $750,000 cap permanent, effective for tax years beginning after December 31, 2025.

So for current debt, the number is $750,000, and it's here to stay—not $1 million. The only way you get the $1 million figure is the grandfather: debt from on or before December 15, 2017. Don't let an old article talk you into the wrong cap.

What if your debt is over the cap? The deduction isn't all-or-nothing—it's prorated. Only the share of your interest attributable to debt within the limit is deductible. Pub. 936's Table 1 worksheet ("Qualified Loan Limit and Deductible Home Mortgage Interest") walks the math, using your average mortgage balance for the year. For a feel: if your loans average $900,000 against the $750,000 cap, only 750 ÷ 900 ≈ 83% of your interest counts—so $30,000 of interest yields roughly a $25,000 deduction, not zero. So "you're over the cap" is rarely a reason to concede the whole deduction—it's a reason to recompute the allowable fraction. More on that below.

The Home-Equity Trap (Also Permanent Now)

This is one of the single most common real-world disallowances, so read it carefully.

A home-equity loan or HELOC—debt secured by your home that isn't acquisition debt—used to get its own deductible allowance. The 2017 law suspended that: for years after 2017, home-equity interest is deductible only if you used the borrowed money to buy, build, or substantially improve the same home that secures the loan. And OBBBA's § 70108 made that suspension permanent too—it did not snap back after 2025 to a deductible $100,000 of home-equity interest.

So the practical rule, now permanent, is blunt:

HELOC used to add a room, redo the kitchen, or build a deck on the home that secures it → that's acquisition debt, deductible (and it counts against your $750,000 cap).
HELOC used to pay off credit cards, buy a car, cover tuition, or take a vacation → nondeductible personal interest. It doesn't matter that the loan is secured by your house. What the money bought is what controls.

If your notice says "this is home-equity debt, not acquisition debt," the entire fight is tracing the proceeds—proving the money went into the home. We come back to how you prove that.

Mortgage Insurance Premiums: Back for 2026 Only

If you pay mortgage insurance—PMI on a conventional loan, or FHA/VA/Rural Housing premiums—there's a fresh wrinkle that depends entirely on the tax year.

The deduction for qualified mortgage insurance premiums (treated as residence interest under § 163(h)(3)(E)) expired after 2021. It was not deductible for tax years 2022 through 2025. OBBBA's § 70108 revived it, beginning with tax year 2026. So:

A 2026 (or later) return: qualified mortgage insurance premiums are deductible again, treated as qualified residence interest—subject to an income phaseout. The deductible amount drops by 10% for each $1,000 ($500 MFS) by which your adjusted gross income (AGI) exceeds $100,000 ($50,000 MFS), and fully phases out at AGI of $110,000 ($55,000 MFS).
A 2022–2025 return: mortgage insurance premiums are correctly disallowed. If you're fighting one of those older years, this revival doesn't help you—it only applies to 2026 forward.

Flag the year clearly when you check your own numbers. This is a deduction that exists on a 2026 return and did not exist on a 2024 one.

Secured, Perfected, and "Two Homes Max"

Two more technical traps decide a surprising number of cases.

The loan must be secured by the residence—and the security interest must be perfected. Pub. 936 is explicit: a secured debt is one where you've signed an instrument that makes your home security for the debt and that is "recorded or otherwise perfected under any state or local law." An unsecured loan produces nondeductible personal interest—even if you spent every dollar on the house. Pub. 936's own wraparound-mortgage example makes the point: where a seller-financed note was never recorded or perfected, it wasn't secured debt, and the buyer "can't deduct any of the interest." Recording matters.

You get two homes, maximum. A "qualified residence" is your principal home plus one other residence you elect—not three, not a portfolio (§ 163(h)(5)(A)). Interest on a third property isn't residence interest.

And that elected second home has to actually be used as a residence. If you rent it out, the § 280A personal-use test applies: you must use it yourself more than the greater of 14 days or 10% of the days it's rented at a fair price. Fall short and it's rental property, not a qualified second home—and the interest becomes a rental expense, governed by different rules. (Rental-mortgage interest routes to How To Deduct Rental and Passive Activity Losses.)

One last item—points. Prepaid interest generally has to be spread over the life of the loan, not deducted all at once (§ 461(g)). The exception: points paid to buy or improve your principal residence can usually be deducted in the year you paid them, if charging points is standard in your area and the amount is customary. Points on a refinance, or on a second home, are normally deducted ratably over the loan term—a common spot where a claimed deduction gets trimmed.

Who Even Gets To Deduct It: The Equitable-Owner Trap

Here's a trap that blindsides people who are paying a real mortgage on a real home they live in: paying the interest isn't enough. You also have to be the owner of the property.

The rule comes from Treas. Reg. § 1.163-1(b): you can deduct mortgage interest only if you're the legal or equitable owner of the home and you actually paid the interest. You don't have to be personally on the note—an equitable (beneficial) owner can deduct—but you have to be an owner in substance, not just someone making payments.

That distinction sank a pro se taxpayer in Puentes v. Commissioner, T.C. Memo. 2013-277. Her brother bought a home near San Francisco and held sole legal title; the mortgage was in his name, and the Form 1098 listed him as the borrower. She lived there for years, and in 2009—when he lost his job—she paid the mortgage, including roughly $28,941 in interest, and deducted it on her own return. The IRS disallowed it and issued a deficiency of $3,809; she petitioned Tax Court.

She lost. She conceded she wasn't the legal owner and argued she was the equitable owner. The court applied a seven-factor "benefits and burdens" test, asking whether she:

had the right to possess the property and enjoy its use, rents, or profits;
had a duty to maintain it;
was responsible for insuring it;
bore the risk of loss;
was obligated to pay its taxes, assessments, and charges;
had the right to improve it; and
had the right to obtain legal title by paying off the balance.

On her facts, almost none of these pointed her way. The court found "no evidence that she had any agreement with [her brother] entitling her to an ownership interest" and that the record was "devoid of any evidence" she was legally obligated to bear any real burden of ownership. Living there and voluntarily paying the mortgage was not enough. (She tried again for the next tax year in Puentes, T.C. Memo. 2014-224—same facts, same result, same answer: you can't fix an ownership failure by re-litigating it.)

The flip side is the lesson worth carrying away: you can win as an equitable owner—but you have to document the benefits and burdens. A written agreement giving you an ownership stake, proof you pay the property taxes and insurance and maintenance, that you bear the risk of loss, and that you have a path to legal title—that's the case the Puentes facts were missing. If the 1098 is in someone else's name, that's not automatically fatal, but it means you have to build the ownership case affirmatively.

Two Owners, Two Caps: The Voss Rule

What if you and another person co-own a home but aren't married—a couple, siblings, partners? Whose $750,000 cap applies?

The answer comes from Voss v. Commissioner, 796 F.3d 1051 (9th Cir. 2015), which reversed the Tax Court below. Two unmarried co-owners had combined mortgage debt over $2.2 million; the IRS tried to limit them to one set of caps between them. The Ninth Circuit held the debt limits apply per taxpayer, not per residence—so each unmarried co-owner gets the full cap. The reasoning: because the statute expressly halves the limits for married people filing separately, Congress must have meant unmarried co-owners each get the whole thing. The IRS formally acquiesced in this result in Action on Decision 2016-02 and now applies the per-taxpayer rule.

Two honest caveats. First, Voss was a represented appellate case, not a pro se win—treat it as precedent the IRS follows, not a template for how easy this is. Second, Voss was decided on the old $1 million figures, so don't over-claim: today the per-taxpayer rule applies to the $750,000 cap, and the home-equity piece of the old rule is mostly gone (home-equity interest is suspended unless the money improved the home). Net result for current unmarried co-owners: each of you can deduct interest on up to $750,000 of acquisition debt—a real benefit, just not "$1.1 million each" anymore. The mirror image: a married couple filing separately each gets half—$375,000.

Investment Interest: § 163(d)

Now the other door. If you borrow to invest—a margin loan to buy stock, for example—the interest is "investment interest," and it escapes the personal-interest knockout. But it comes with its own ceiling.

You can deduct investment interest only up to your "net investment income" for the year (§ 163(d)(1)). Pay $8,000 of margin interest but have only $5,000 of net investment income, and your deduction is capped at $5,000.

The good news is what happens to the rest. The excess carries forward—indefinitely (§ 163(d)(2)). The $3,000 you couldn't use this year becomes investment interest "paid" next year, where you try again. So an over-the-cap disallowance here is usually a timing cost, not a permanent loss. You report all of this on Form 4952 (Investment Interest Expense Deduction), and it flows to Schedule A.

The Election Trade-Off

What counts as "net investment income"? Roughly, your investment income (interest, ordinary dividends, royalties) minus your investment expenses. But here's the catch that surprises people: it generally excludes your net capital gain and qualified dividends—the income taxed at the preferential 0/15/20% rates (§ 163(d)(4)(B)). So a year where your gains were all long-term capital gains can leave you with very little "net investment income" to deduct interest against.

There's a lever, but it's a trade-off. You can elect to treat some of your qualified dividends or net capital gain as ordinary investment income—which raises your net investment income and frees up more interest deduction. The cost: the amount you elect loses the low capital-gains rate and is taxed as ordinary income. Per Pub. 550, "before making either choice, consider the overall effect on your tax liability." In plain terms: you give up a low rate on some gain to unlock an interest deduction, and it only pays off if the deduction is worth more than the rate hike. Run both ways before you elect.

A worked shape, adapted from Pub. 550: investment income of $10,000 minus investment expenses of $3,200 = net investment income of $6,800. Against $8,000 of investment interest expense, your deduction is capped at $6,800, and the remaining $1,200 carries forward to next year. The cap costs you nothing permanently—it just defers $1,200 of deduction.

Don't Confuse This With § 265—That One's Fatal

Keep one neighbor strictly separate, because it has a much worse result. Interest on debt used to buy or carry tax-exempt bonds—like borrowing to hold municipal bonds whose interest is tax-free—is disallowed entirely under § 265(a)(2). Pub. 550 puts it plainly: "You cannot deduct interest expenses you incur to produce tax-exempt income."

This is the distinction that matters: § 163(d) caps your investment interest (excess carries forward); § 265(a)(2) deletes it. No cap to climb back into, no carryforward—it's simply gone. If the IRS recharacterizes your margin loan as one used to carry tax-exempt bonds, the answer isn't "deduct up to net investment income"—it's zero. Different statute, much worse outcome. (And it cuts the other way: if the IRS wrongly applies § 265 to a loan used for taxable investments, that's an error to push back on.)

How To Verify the IRS's Number

Before you concede a dollar, pin down exactly what the IRS used and rebuild your own number. Every step here is something you can do yourself.

Pull your Wage & Income transcript. This shows the actual Form 1098 your lender filed under your SSN—the mortgage interest figure the IRS's computer matched against your return—along with the 1099-INT, 1099-DIV, and 1099-B data that feed the investment-interest computation. It's free through IRS.gov Get Transcript; you don't need a practitioner. See How To Get and Read Your IRS Transcripts.

Then work down the list of the recurring ways these disputes get framed:

"Your loan wasn't secured." Produce the recorded mortgage, deed of trust, or land contract. Unsecured (or unperfected) = personal interest.
"It's home-equity debt, not acquisition debt." Post-OBBBA this is fatal unless you trace the money to buying, building, or substantially improving the home. Bring the closing statement, contractor invoices, and the loan application showing the use of funds.
"You're over the cap." Recompute the prorated allowable interest with Pub. 936's Table 1 worksheet, using your average mortgage balance for the year (average of the first and last balance, or interest paid divided by the rate, or the lender's monthly statements). Don't accept a flat "all disallowed" when only the slice over the limit should be cut.
"Form 1098 doesn't match." Cross-check the lender's 1098 against what you claimed. If you claimed more than the 1098 shows, be ready to substantiate the extra. If your loan is seller-financed with no 1098, the interest is still deductible—but you must report the payee's name, address, and SSN/EIN on Schedule A, and your canceled checks and amortization schedule carry the day. (The Puentes pattern—a 1098 in someone else's name—is the classic version of this.)
"Too many homes." Only your principal residence plus one elected second home qualify.
"Your second home is really a rental." Apply the § 280A 14-day/10% personal-use test.
Investment interest: recompute net investment income, weigh the § 163(d)(4)(B) election, and confirm the IRS hasn't mistakenly applied § 265 (tax-exempt) to a taxable-investment loan.

Know where the burden sits. In Tax Court the IRS's notice is presumed correct, and a deduction is "a matter of legislative grace"—you have to affirmatively prove your right to it. That principle runs from New Colonial Ice Co. v. Helvering, 292 U.S. 435 (1934) and INDOPCO, Inc. v. Commissioner, 503 U.S. 79 (1992) through Welch v. Helvering, 290 U.S. 111 (1933), the case the Puentes court cited for it. IRC § 7491 can shift the burden to the IRS, but only after you produce credible evidence and meet your recordkeeping duties—so plan on the burden being yours and win it with documents.

What You're Really Facing: The Full Exposure Picture

An interest-disallowance notice is rarely just the headline number, but it's also rarely as bad as it looks. Build the whole picture honestly—and understand the one feature that makes mortgage-interest disputes different.

The Itemize-vs-Standard Flip (This Is the Big One)

Mortgage interest is an itemized deduction. That changes the math in your favor. You only got a tax benefit from it in the first place to the extent your total itemized deductions beat the standard deduction. So losing it may simply drop you back to the standard deduction—and the real cost is the difference, not the gross interest disallowed.

Here's an illustrative example (round numbers; label it illustrative). A single filer, AGI $90,000, in the 22% bracket, claimed $20,000 of itemized deductions—$14,000 of it home-mortgage interest from a HELOC the IRS now disallows because the money paid off credit cards (nondeductible home-equity interest). Strip out the $14,000, and only $6,000 of itemized deductions survive.

But the 2026 standard deduction for a single filer is $16,100 (per IRS Rev. Proc. 2025-32). Since $6,000 is well below $16,100, the filer simply takes the standard deduction. The added taxable income isn't the full $14,000—it's the difference between $16,100 and the $6,000 of surviving itemized deductions, or about $10,100. At 22%, the real tax hit is roughly $2,222—not the $3,080 you'd get by panicking and multiplying the full $14,000 by 22%.

So before you assume the worst: compute the delta, not the gross. The standard deduction is a floor that absorbs part of the loss.

For an investment-interest disallowance, remember it's usually a timing cost: the over-the-cap piece carries forward (§ 163(d)(2)), so $1,200 disallowed this year is $1,200 of deduction next year—painful for cash flow, not a permanent loss. Contrast that with a § 265 disallowance, which is permanent.

Penalty, Interest, and the Statute

The 20% accuracy penalty. A § 163 disallowance often rides with a 20% accuracy-related penalty under § 6662 for negligence or a substantial understatement. There's a real defense: reasonable cause and good faith under § 6664(c)—you relied on a Form 1098, a settlement statement, or a preparer, and honestly believed the interest qualified. The IRS also has to show written supervisory approval of the penalty under § 6751(b). The full playbook lives in How To Fight the IRS Accuracy Penalty—link, don't re-derive.
Interest. Interest runs on any deficiency from the original due date of the return until it's paid (§ 6601), compounds daily, and—unlike the penalty—generally can't be abated for reasonable cause. One more reason to fix the number fast. See How Interest Works on Your IRS Tax Debt.
Can the IRS still do this? Generally the IRS has 3 years from when you filed to assess more tax. Check the dates on your notice against Understanding IRS Statutes of Limitations.

Because the penalty and interest are computed on the deficiency, every dollar you knock off the disallowance shrinks everything below it too.

Put it together for that example. Take the ~$2,222 of added tax from the HELOC disallowance above. A 20% accuracy penalty, if it sticks, adds about $444; interest accrues on the deficiency from the return's original due date until you pay—often a few hundred dollars more across the year or two a dispute runs. So the realistic all-in is roughly $2,700 plus accruing interest—well under the ~$3,080 you'd fear from multiplying the gross, and a world away from the $14,000 headline. That total—not the headline disallowance—is the number to weigh against the cost and odds of fighting.

If the IRS is right and you can't pay. Sometimes the interest really wasn't deductible—a HELOC that paid off cards, a year before MIP came back, a loan you couldn't trace to the home. Conceding the tax doesn't mean writing one check today. The IRS has 10 years to collect from the date it assesses, and there are structured ways to carry the bill: an installment agreement to pay over time, currently not collectible status if you genuinely can't pay anything right now, or an offer in compromise to settle for less. How To Resolve Your IRS Tax Debt maps the options.

The Appeals Fork Before Tax Court

You usually don't have to wait for a notice of deficiency to fight. If you're still at the examination or 30-day-letter stage—a proposed change, no formal deficiency yet—IRS Appeals is the cheaper, faster fork. File a protest and put your recorded mortgage, 1098, and tracing documents in front of an independent reviewer who weighs the "hazards of litigation." An interest dispute, being so document-driven, is exactly the kind of case Appeals settles. See How To Request an IRS Appeals Conference.

Once the Notice of Deficiency (the "90-day letter") issues, the 90 days Tax Court clock starts (§ 6213(a); 150 days if the notice was addressed outside the US), and it cannot be extended. See You Just Got a 90-Day Letter From the IRS.

What To Do Now

If an interest-disallowance notice is in front of you, here's the sequence:

Identify which notice you have—and whether a clock is running. Tell them apart at a glance: a 90-day letter is titled "Notice of Deficiency" and prints a hard last date to petition—the clock is running (90 days, 150 days if addressed outside the US) and it cannot be extended; a CP2000 is an automated under-reporter proposal with a reply-by date and no Tax Court clock yet; a 30-day letter (examination report) invites you to Appeals. Not sure which you're holding? Common IRS Notices and Letters helps you classify it.
Pull your Wage & Income transcript and confirm the exact Form 1098 (and 1099) data the IRS has under your SSN.
Figure out which fight it is. Acquisition vs. home-equity (trace the money)? Over the cap (recompute the prorated share)? Ownership (build the equitable-owner case)? Investment interest over net investment income (recompute, weigh the election)? Each has a different answer.
Gather the documents that win it: the recorded mortgage or deed of trust, the closing statement, contractor invoices showing where HELOC money went, every Form 1098, and—for investment interest—Form 4952 and your brokerage statements.
Run the exposure math honestly. For mortgage interest, compute the delta against the standard deduction (2026 single: $16,100), not the gross. For investment interest, remember the over-cap piece carries forward.
Choose your fork. A documented protest to IRS Appeals can settle a fact-specific interest case before any petition. If you petition Tax Court, filing is $60 (with a waiver available). Many single-year interest disputes fall under the $50,000 ceiling for the simpler "S case" procedure. See How To File Your Tax Court Petition.
Expect to resolve before trial. Most (76%) of Tax Court cases close by formal settlement. The IRS routinely concedes once you put a recorded mortgage, a matched 1098, and a clean tracing trail in front of it. See How To Settle Your Tax Court Case.

One more thing to plan for: a change to your federal interest deduction usually changes your state tax too, since most states pick up a federal adjustment automatically.

Around 89% of petitioners represent themselves, though the win rate trails represented petitioners (about 12% pro se versus about 23% represented in the most recent NTA data). And if your income is at or below 250% of the poverty line and your dispute is at or below $50,000, a Low Income Taxpayer Clinic may take your Tax Court case for free.

Resources

Statutes and regulations:

IRS guidance, forms, and publications:

Companion articles on TaxCourtHelp:

Cases cited:

This article is for informational purposes only and does not constitute legal or tax advice. For advice specific to your situation, consult a qualified tax professional or attorney.