IRS Publication 936: Home Mortgage Interest Deduction
IRS Publication 936 explains the rules for deducting mortgage interest on your tax return — and the landscape shifted dramatically after the Tax Cuts and Jobs Act. The deduction is now capped at interest on $750,000 of acquisition debt. Home equity loan interest is only deductible if the loan was used to buy, build, or improve the home. Points on a refinance are amortized over the loan term. And if your total mortgage debt exceeds the limit, only a fraction of your interest is deductible. Getting the calculation wrong can mean losing thousands in legitimate deductions or triggering an IRS notice.
How the Mortgage Interest Deduction Works
Step 1: Determine Your Qualified Residence Loans
Qualified residence loans include mortgages on your main home and one second home. The IRS caps the deduction at interest on $750,000 of combined acquisition debt for loans originated after December 15, 2017 ($375,000 for married filing separately). Loans originated on or before that date are grandfathered at the pre-TCJA limit of $1,000,000 ($500,000 MFS). A second home must have sleeping, cooking, and toilet facilities — it can be a traditional house, condo, mobile home, boat, or RV.
Step 2: Classify Your Debt — Acquisition vs. Home Equity
Acquisition debt is money borrowed to buy, build, or substantially improve a qualified residence. Home equity debt is borrowing secured by the home but used for other purposes. Under TCJA, only acquisition debt interest is deductible. A home equity loan used to add a new bathroom is acquisition debt (deductible). The same loan used to pay off credit cards is not deductible. The IRS applies tracing rules: follow the money to where it was spent, regardless of what the loan is called.
Step 3: Compute the Deductible Percentage if Over the Limit
If your total mortgage debt exceeds the applicable limit ($750,000 or $1M), divide the limit by your average mortgage balance for the year. Multiply total mortgage interest paid by that percentage. Average mortgage balance can be computed using the first-of-month balance method (sum of balances on the first of each month divided by the number of months the loan was outstanding) or the interest rate method (total interest divided by the stated annual rate).
Step 4: Report on Schedule A — Itemized Deductions
The mortgage interest deduction is claimed on Schedule A (Form 1040), line 8a for home mortgage interest and line 8b for deductible points not reported on Form 1098. Your lender will send Form 1098 showing the interest you paid. Remember: you only benefit from itemizing if your total itemized deductions exceed the standard deduction ($14,600 single / $29,200 married filing jointly for 2024). Many taxpayers who previously itemized no longer benefit due to the higher TCJA standard deduction.
Step 5: Handle Special Situations
Rental use of your home: if you rent out part of your home or your second home, you must allocate mortgage interest between personal (Schedule A) and rental (Schedule E) use based on the number of days used for each purpose. Cooperative housing: tenant-stockholders in a co-op can deduct their share of the cooperative's mortgage interest, reported on Form 1098 from the cooperative corporation. Mortgage insurance premiums: treated as deductible qualified residence interest through the current tax year, subject to phase-out for AGI over $100,000.
Key Rules for Special Scenarios
Points on a Refinance — Amortize, Don't Deduct Upfront
Points paid to refinance a mortgage must be amortized over the life of the loan, not deducted in the year paid. If you pay $3,000 in points on a 30-year refinance, you deduct $100 per year ($3,000 / 360 months x 12). If you refinance again with a different lender, you can deduct any remaining unamortized points from the first refinance in full that year. If you refinance with the same lender, the remaining points from the old loan are amortized over the new loan term.
Grandfathered Pre-TCJA Loans
Loans taken out on or before December 15, 2017 retain the pre-TCJA $1,000,000 acquisition debt limit ($500,000 MFS). Refinancing a grandfathered loan preserves the grandfathered status up to the original loan's principal balance — but any additional cash-out borrowing above the original balance is subject to the $750,000 limit. The grandfathered status is lost if the refinanced loan exceeds the remaining principal of the original loan at the time of refinancing.
Second Home — Personal Use vs. Rental
If you use your second home as a residence (personal use exceeds the greater of 14 days or 10% of rental days), all mortgage interest is deductible as qualified residence interest on Schedule A, and you do not report rental income if rented 14 days or fewer. If you rent it more extensively, you must allocate interest between personal and rental use. If personal use is minimal (less than 14 days or 10% of rental days), the property may be treated as a rental property and all interest reported on Schedule E — but then the $750,000 limit does not apply, and interest is a rental expense.
Cooperative Housing Deductions
If you own shares in a cooperative housing corporation, you can deduct your proportionate share of the corporation's mortgage interest and real estate taxes. The co-op will issue Form 1098 showing your share of deductible mortgage interest. To qualify, you must be a tenant-stockholder, the cooperative must own the building, and the interest must be on debt secured by the cooperative's building or land. The same $750,000 acquisition debt limit applies to your share of the co-op's mortgage.
Mortgage Insurance Premiums
Qualified mortgage insurance premiums paid or accrued in connection with acquisition debt are treated as deductible qualified residence interest. This applies to private mortgage insurance (PMI) on conventional loans, FHA mortgage insurance premiums, VA funding fees, and USDA guarantee fees. The deduction phases out by 10% for each $1,000 of AGI above $100,000 (so it is fully phased out at $109,000 for single filers and $109,000 for married filing jointly through the current allocation). This provision has been repeatedly extended by Congress but is subject to annual renewal.
Refinancing a Grandfathered Loan — Preserving the $1M Limit
When refinancing a pre-December 15, 2017 loan, the new loan retains grandfathered status ONLY up to the principal balance of the old loan at the time of refinancing. For example, if your original grandfathered loan had a remaining balance of $600,000 and you refinance for $700,000 (cash-out of $100,000), then $600,000 is grandfathered at the $1M limit and the extra $100,000 is subject to the $750,000 limit. Any home equity debt from cash-out must be traced to its use — if used for home improvements, it becomes acquisition debt; if used for personal expenses, it is not deductible.
Calculating the Deduction When Your Mortgage Exceeds the Limit
The Formula
Deductible interest = Total mortgage interest paid x (Applicable loan limit / Average mortgage balance). If your average balance is $900,000 and you are subject to the $750,000 limit, you can deduct 83.3% of your total mortgage interest. For grandfathered loans at the $1M limit with a $900,000 average balance, 100% is deductible because the balance is under the limit.
Average Balance — First-of-Month Method
Sum the outstanding mortgage principal on the first day of each month the loan was outstanding, then divide by the number of months. For a loan originated on March 15, count March through December (10 months). If your principal was $800,000 at origination and you paid it down to $785,000 by year-end, compute each month's first-of-month balance individually — do not simply average the starting and ending balance.
Average Balance — Interest Rate Method
Alternative method: divide the total interest paid for the year by the loan's stated annual interest rate. If you paid $48,000 in interest on a 6% mortgage, your average balance is $800,000 ($48,000 / 0.06). This method only works if the interest rate was constant all year and no new borrowing or principal payments occurred mid-year that would distort the computation.
Multiple Loans — Combined Limit
The $750,000 (or $1M) limit applies to the COMBINED total of all mortgages on your main home and second home. If you have a $600,000 mortgage on your primary residence and a $300,000 mortgage on a second home, your total is $900,000 — $150,000 over the $750,000 limit. You would deduct 83.3% ($750,000/$900,000) of your total mortgage interest from both loans combined.
Common Mistakes and IRS Audit Triggers
Deducting Home Equity Interest on Personal Expenses
This is the most common post-TCJA mistake. Taxpayers continue deducting interest on home equity loans and lines of credit used for vacations, cars, tuition, or debt consolidation. The IRS Form 1098 from the lender shows total mortgage interest paid without distinguishing between deductible and non-deductible portions. It is the taxpayer's responsibility to determine which portion is deductible based on how the loan proceeds were used. The IRS regularly reviews Schedule A deductions against Form 1098 and sends CP2000 notices when the numbers do not reconcile.
Forgetting to Amortize Refinance Points
Many taxpayers deduct all refinance points in the year of the refinance — the same rule that applies to purchase mortgages. This is incorrect. Refinance points must be amortized over the loan term. Deducting them all upfront is a red flag for the IRS, especially when the dollar amount is large. Calculate the annual amortization amount: points paid divided by the number of months in the loan term, multiplied by 12.
Claiming Interest on More Than Two Homes
The mortgage interest deduction is limited to your main home and ONE second home. If you own three homes with mortgages, interest on the third is not deductible as qualified residence interest. If the third home is a rental property, its mortgage interest is a rental expense on Schedule E — but then it is not qualified residence interest and does not count toward the $750,000 limit. You can change which home is designated as your second home from year to year to maximize the deduction.
Exceeding the Limit Without Realizing It
Taxpayers with pairs of mortgages — a first and second mortgage on the same property, or mortgages on two homes — often exceed the combined limit without realizing it. The IRS expects you to compute the deductible percentage if your total exceeds the limit, not simply deduct all interest shown on Form 1098. Failing to prorate the deduction when the combined balance exceeds the limit results in overstated Schedule A deductions, which the IRS can identify through automated matching programs comparing Form 1098 data to Schedule A entries.
Misclassifying Rental Use of a Second Home
Homeowners who rent their second home through Airbnb, VRBO, or similar platforms face complex allocation rules. If you classify the property as a residence (personal use over 14 days or 10% of rental days), the mortgage interest allocated to personal use is deductible on Schedule A, and the rental portion on Schedule E. If you classify it as a rental property (limited personal use), all interest is on Schedule E — but you may lose the qualified residence interest characterization, which can matter for AMT and other interactions. Keep a detailed log of personal-use days vs. rental days.
Not Tracking Mortgage Balance Throughout the Year
The deductible percentage depends on your average mortgage balance for the year, not the year-end balance. Taxpayers who make a large principal payment in December and compute the limitation on the reduced balance will overstate their deduction. The IRS expects you to use the average balance — computed properly using either the first-of-month or interest rate method. Making a large principal payment late in the year has minimal impact on the deductible percentage because the average balance remains high.