13.1 Home Mortgage Interest Deduction ($750K Limit) & Investment Interest Expense (Form 4952)

Qualified Residence Interest Framework (IRC §163(h))

Under IRC §163(h), personal interest is generally nondeductible. However, Congress provides a major exception for qualified residence interest paid on mortgages secured by a taxpayer's personal homes. Reported on Schedule A (Form 1040), Lines 8a–8e, this deduction is subject to strict statutory loan limits and debt-tracing requirements.

Definition of a Qualified Residence

A taxpayer can deduct mortgage interest on a maximum of two homes:

1. Principal Residence: The taxpayer's main home (where they live most of the time).
2. One Designated Second Residence: A vacation home, cabin, condominium, mobile home, or boat with basic living accommodations (sleeping space, toilet, and cooking facilities).
   • Rental Property Rule: If the second home is rented out during the year, the taxpayer must use it personally for the greater of: (a) 14 days, or (b) 10% of the number of days it is rented at fair market value (under IRC §280A). If personal use falls below this threshold, the property is treated as rental property, and mortgage interest must be allocated to Schedule E rather than Schedule A.

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Acquisition Indebtedness Dollar Caps & Grandfathered Debt

To be deductible, mortgage debt must constitute acquisition indebtedness—debt incurred in acquiring, constructing, or substantially improving a qualified residence, secured by that residence.

Refinancing Grandfathered Debt

When a taxpayer refinances grandfathered debt (incurred on or before Dec 15, 2017), the new mortgage retains grandfathered status up to the remaining principal balance of the old loan immediately prior to refinancing. However, any "cash-out" amount that exceeds the old loan principal is treated as new post-2017 debt subject to the $750,000 limit and must be traced to substantial home improvements to be deductible.

Proration Formula for Excess Indebtedness

When a taxpayer's total acquisition debt exceeds the statutory limit, mortgage interest cannot be deducted in full. The allowable deduction is calculated using the following statutory formula:

$\text{Deductible Fraction} = \frac{\text{Statutory Debt Limit (\$750,000 or \$1,000,000)}}{\text{Total Average Debt Principal Balance during the Year}}$ $\text{Allowable Mortgage Interest} = \text{Total Interest Paid} \times \text{Deductible Fraction}$

Calculation Walkthrough: In 2024, Rebecca purchases a principal home and takes out a $1,000,000 first mortgage. During 2025, her average principal balance is $1,000,000 and she pays $60,000 in interest. Because her loan originated after Dec 15, 2017, her debt is capped at $750,000.

1. Deductible Fraction: $750,000 / $1,000,000 = 75.0%.
2. Allowable Schedule A Deduction: $60,000 \times 75.0% = $45,000.
3. Nondeductible Personal Interest: $15,000.

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Home Equity Loans and HELOCs: The Use-of-Proceeds Rule

Prior to 2018, taxpayers could deduct interest on up to $100,000 of home equity debt regardless of how the proceeds were spent. Under current law (the TCJA rule that OBBBA made permanent, along with the $750,000 acquisition-debt limit):

• Strict Tracing Required: Interest on a Home Equity Line of Credit (HELOC) or second mortgage is deductible ONLY IF the borrowed funds are used to buy, build, or substantially improve the qualified residence securing the loan.
• Personal Use Disallowed: If HELOC funds are used to pay off credit card balances, pay college tuition, purchase an automobile, or fund personal vacations, the interest is 100% nondeductible personal interest.
• Cross-Collateralization Trap: If a taxpayer takes out a HELOC on their primary residence to build an addition on their vacation cabin, the interest is nondeductible because the loan proceeds were not used to improve the specific residence that secures the debt!

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Points and Loan Origination Fees (IRC §461(g))

Points (also called loan origination fees, maximum loan charges, or discount points) represent prepaid interest paid by a borrower to secure a lower mortgage interest rate. One point equals 1% of the loan amount.

Immediate Deduction vs. Ratable Amortization

1. Purchase of Principal Residence (Immediately Deductible): Under IRC §461(g)(2), points paid on a loan used to buy or build a principal residence can be deducted in full in the year paid if all statutory criteria are met:
   • The loan is secured by the taxpayer's principal residence;
   • Paying points is an established business practice in the geographic area;
   • The points paid do not exceed customary local charges;
   • The points are computed as an exact percentage of the principal;
   • The taxpayer provided unborrowed personal funds at closing (cash, earnest money, down payment) at least equal to the points charged.
2. Refinancing Points (Ratable Amortization): Points paid to refinance an existing mortgage or paid on a second home cannot be deducted immediately. They must be amortized ratably over the entire term of the loan.
   • Example: Paying $3,600 in points to refinance a 30-year (360-month) mortgage permits a deduction of $10 per month ($120 per year).
   • Early Payoff / Re-refinancing Acceleration: If the refinanced loan is paid off early (through home sale or refinancing with a different lender), any remaining unamortized points can be deducted in full in the year of payoff. However, if the mortgage is refinanced again with the same lender, the remaining points cannot be deducted immediately and must be amortized over the term of the new loan.
3. Seller-Paid Points: Points paid by the seller are treated as if paid directly by the buyer using unborrowed funds. The buyer deducts the points on Schedule A, but must reduce their tax basis in the home by the amount of seller-paid points.

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Investment Interest Expense (IRC §163(d) & Form 4952)

Under IRC §163(d), non-corporate taxpayers may deduct investment interest expense—interest paid or accrued on debt incurred to purchase or carry property held for investment (such as margin loans to purchase corporate stock or debt used to buy raw investment land).

The Net Investment Income (NII) Ceiling

Investment interest expense is deductible only to the extent of Net Investment Income (NII) for the taxable year. It cannot be used to offset wage income, active business profit, or create a tax loss.

$\text{Net Investment Income} = \text{Gross Investment Income} - \text{Deductible Investment Expenses (excluding interest)}$

Components of Gross Investment Income

• Included by Default: Taxable interest income, nonqualified (ordinary) dividends, annuities, royalties, and net short-term capital gains from the disposition of investment assets.
• Excluded by Default: Qualified dividends (IRC §1(h)(11)) and Net Capital Gain (excess of net long-term capital gain over net short-term capital loss). Because these items enjoy preferential capital gains tax rates (0%, 15%, or 20%), Congress excludes them from Net Investment Income to prevent a double tax benefit.

The Section 163(d)(4)(B) Ordinary Rate Election

A taxpayer with substantial investment interest expense and excess capital gains/qualified dividends may make an affirmative election under IRC §163(d)(4)(B) on Form 4952:

• The taxpayer elects to include all or a portion of their qualified dividends or net long-term capital gains in Net Investment Income.
• Trade-Off: The elected amount unlocks an equivalent dollar amount of investment interest deduction, but the elected dividends/gains lose their preferential tax rates and must be taxed at ordinary income tax rates!

Indefinite Carryforward Rule

Any investment interest expense disallowed in the current tax year because it exceeds Net Investment Income is carried forward indefinitely to future tax years. It does not expire, and retains its character on Form 4952 in subsequent years.

Excluded Categories from §163(d):

• Municipal Bond Debt (IRC §265): Interest on debt used to purchase or carry tax-exempt municipal bonds is strictly disallowed.
• Passive Activity Interest (IRC §469): Interest incurred in rental properties or passive partnerships is subject to passive activity loss rules, reported on Schedule E, not Form 4952.