Mortgage Interest Deduction Post-TCJA: The $750K Cap That Hits Bay Area Buyers

If you bought a Palo Alto, Mountain View, or Cupertino home in the last few years, you have almost certainly run into the silent ceiling that the Tax Cuts and Jobs Act dropped on the mortgage interest deduction. The federal rule is simple to state and brutal in practice: interest is deductible only on the first $750,000 of acquisition debt on loans originated December 16, 2017 or later. A typical Bay Area buyer borrows two to three times that. The math difference between "all interest deductible" and "37.5% of interest deductible" is roughly the price of a midsize sedan every year.

This post walks the post-TCJA rules in detail: how the $750,000 cap works, who still has the grandfathered $1,000,000 cap, what happens when you refinance, the HELOC trap that catches homeowners who used a home equity line to pay off credit cards, the points rule, the unmarried co-owner workaround that doubles the cap, and how California conforms.

For the loan side of any purchase or refinance you handle that with a mortgage broker or lender. What we do at Silicon Valley Tax is model the after-tax cost so you know what the deal actually looks like on your return.

The $750,000 Cap and the Statute Behind It

IRC §163(h)(3) allows individuals to deduct interest on "qualified residence interest" if they itemize on Schedule A. The Tax Cuts and Jobs Act of 2017 amended this section to limit acquisition indebtedness to $750,000 for loans originated on or after December 16, 2017. For married filing separately, the cap is $375,000 each.

"Acquisition indebtedness" is debt that meets three tests:

1. Incurred to buy, build, or substantially improve a qualified residence (primary home or one second home),
2. Secured by that residence, and
3. Not in excess of the home's adjusted basis.

The cap is a cap on the loan principal, not on the interest itself. If your acquisition debt is below $750,000, all of your mortgage interest is potentially deductible (subject to the standard-vs-itemize choice). If your debt is above $750,000, you can deduct only the share of interest attributable to the first $750,000 of average balance. The math is a ratio: deductible interest = total interest paid × ($750,000 ÷ average loan balance).

The TCJA cap was originally scheduled to sunset after 2025 and revert to the pre-TCJA $1,000,000 ceiling. The One Big Beautiful Bill Act (OBBBA, Public Law 119-21, signed July 4, 2025) extended the $750,000 acquisition-debt cap rather than letting it expire. Plan for the $750,000 cap to remain in effect for 2026 and beyond unless Congress acts again. The IRS's primary reference for homeowners on these rules is Publication 936, Home Mortgage Interest Deduction.

The Grandfathered $1,000,000 Cap

Loans originated before December 16, 2017 retain the prior-law $1,000,000 cap ($500,000 if MFS). This grandfathering is loan-specific, not borrower-specific, and it survives a refinance under one important condition: when you refinance a grandfathered loan, the new loan keeps the $1,000,000 cap only up to the remaining balance of the original loan at the time of refinance. Any amount borrowed above that balance falls under the $750,000 regime as either acquisition debt or home equity debt depending on use.

There was also a binding-contract carveout: if you had a written binding contract before December 15, 2017 to buy a primary residence and closed before April 1, 2018, the $1,000,000 cap applies to that purchase loan.

If you are sitting on a 3% pre-2017 mortgage, the grandfathered cap is one more reason not to touch it. Refinancing into a 6.5% market may be unattractive on the rate alone, and losing the $1M cap on the spread between your original balance and your new balance can make it worse.

Bay Area Math: What the Cap Actually Costs

Make the example concrete. A typical Palo Alto purchase in 2025:

• Purchase price: $2,500,000
• Down payment: $500,000 (20%)
• Mortgage: $2,000,000 at 7% (30-year fixed)
• Year-one interest paid: approximately $140,000

Run the §163 ratio. Deductible interest = $140,000 × ($750,000 ÷ $2,000,000) = $52,500. The remaining $87,500 of interest is non-deductible.

At a 37% federal marginal rate plus 9.3% California, the lost deduction translates to roughly $40,600 in tax savings forgone in year one alone. Over the first five years of the loan, that compounds into low-six-figure territory. Buyers who pencil out "I get to write off my mortgage interest" without running the cap math are usually planning on tax savings that won't actually appear.

This is the kind of after-tax modeling we run for tech earners in our tech employee tax practice. The mortgage decision is rarely just a rate decision; it is a cash-flow plus tax-deduction plus alternative-investment decision.

The HELOC Trap

Pre-TCJA, interest on home equity loans and HELOCs was deductible up to $100,000 regardless of how the proceeds were used. The TCJA killed that. Under the post-2017 rules in IRC §163(h)(3)(F), interest on home equity debt is deductible only if the proceeds were used to buy, build, or substantially improve the home that secures the loan. The use test, not the loan's name, controls.

The practical effect:

• HELOC drawn to fund a kitchen remodel on the home that secures it: deductible (treated as acquisition debt, counts against your $750,000 cap).
• HELOC drawn to pay off credit cards, fund a wedding, buy a car, pay tuition, or invest in the market: not deductible under current law (the post-TCJA non-acquisition disallowance was extended by OBBBA 2025).
• HELOC drawn to renovate a rental property you own (not the home securing the HELOC): not deductible as home mortgage interest, though it may be deductible against the rental on Schedule E if traced correctly.

The interest tracing rules are unforgiving. If you commingle HELOC proceeds with other money, you can lose the deduction entirely. Open a separate account for the HELOC draw, pay the qualifying expenses directly from it, and keep contemporaneous records. The IRS does not award credit for "I meant to."

What "Substantially Improve" Actually Means

The substantially-improve standard borrows from the repair-vs-improvement framework in the §263 regulations. An improvement adds to the home's value, prolongs its useful life, or adapts it to a new use. Examples that qualify:

• Kitchen or bathroom remodel
• Room addition or ADU construction
• New roof, HVAC system, or solar installation
• Foundation work, structural reinforcement, seismic retrofit
• Pool, deck, or major landscaping that increases property value

Examples that do not qualify (these are repairs and maintenance):

• Painting (interior or exterior)
• Replacing broken windows or appliances on a like-for-like basis
• Routine roof patching, gutter cleaning, plumbing fixes
• Standard yard maintenance

Document everything. Save contractor invoices, building permits, before-and-after photos, and a brief project summary. If the IRS challenges your HELOC deduction five years from now, the burden of proof is on you. Receipts win, memories lose.

Refinancing: Keeping (and Losing) Acquisition-Debt Character

Refinancing your acquisition debt does not, by itself, change its character. A rate-and-term refinance of a $1.2M acquisition loan into a new $1.2M loan keeps the original loan's status (and, if grandfathered, the $1M cap up to the remaining old balance). The clock does not reset.

Cash-out refinances are where it gets tricky. The new loan is split into two pieces:

1. Up to the remaining balance of the old loan: retains acquisition-debt status (and, if grandfathered, the $1M cap up to that balance).
2. The cash-out amount above the old balance: treated as home equity debt. Deductible only if the cash was used to substantially improve the residence. Used for anything else, the interest on the excess is non-deductible.

Example. You have a grandfathered 2016 loan with a $900,000 balance. You refinance into a new $1,200,000 loan, taking $300,000 cash out to fund a kitchen remodel. The $900,000 portion is acquisition debt under the old $1M cap; the $300,000 cash-out is also acquisition debt because the proceeds substantially improve the home, but it falls under the post-TCJA $750,000 framework. Total qualifying acquisition debt: $900,000 (grandfathered) + however much room is left in your $750,000 post-TCJA cap, applied to the $300,000 cash-out. The interplay between the two regimes when both are in play on the same property is exactly the kind of return-prep moment where a CPA earns their fee.

If the $300,000 cash-out had funded credit-card payoff instead, none of it would be deductible.

The Points Rule (IRC §461(g))

IRC §461(g) and the IRS safe harbor in Rev. Proc. 94-27 together govern when "points" (loan origination fees expressed as a percentage of the loan amount) are deductible.

For points paid on a loan to purchase a primary residence:

• Deductible in full in the year of payment, provided the standard safe-harbor conditions are met (loan secured by primary home, the points are an established business practice in the area, the amount is reasonable, the points are computed as a percentage of the loan, the points are clearly identified on the settlement statement, and the buyer pays the points directly with funds not borrowed from the lender).
• Reported on Form 1098 by the lender; you deduct on Schedule A.

For points paid on a refinance or on a second home:

• Must be amortized over the life of the new loan. On a 30-year refi with $20,000 in points, you deduct $20,000 ÷ 360 months × 12 months = $667 per year.
• If you pay off (or refinance again) the loan before the points are fully amortized, the unamortized balance is deductible in full in the year of payoff.

The trickle-deduction reality on refinance points is one reason a lot of buyers should think twice about buying down rate via points: the immediate cash outlay is rarely justified by a $667 annual deduction over 30 years.

The Voss Workaround for Unmarried Co-Owners

This is the single most valuable planning angle in this post for unmarried Bay Area couples buying together. In Voss v. Commissioner, 796 F.3d 1051 (9th Cir. 2015), the Ninth Circuit held that the home mortgage interest deduction limits apply per taxpayer, not per residence. The IRS acquiesced to this holding in AOD 2016-02 and applies the per-taxpayer rule nationwide.

The consequence: two unmarried co-owners of the same home each get their own $750,000 cap. Together they can deduct interest on up to $1,500,000 of acquisition debt. Married couples filing jointly share one $750,000 cap. Two engineers buying together unmarried can shelter twice the interest two engineers buying together married can.

This is real money in Bay Area dollars. On a $1.5M acquisition loan, an unmarried couple with title and liability split appropriately can deduct effectively 100% of the interest; a married couple deducts roughly half. The tax savings differential at high brackets can exceed $15,000 per year.

The structure has to be done right:

• Both individuals must be on title (joint tenancy or tenancy in common).
• Both must be obligated on the note (co-borrowers on the mortgage, not one as borrower and the other as guarantor).
• Each individual deducts interest on their own Schedule A based on what they actually paid.
• Form 1098 will arrive in one name (the primary borrower); the non-primary co-owner deducts as a "qualified payer" by attaching a statement showing the lender's name, amount of interest paid, and the primary borrower's name and address.

If you are unmarried, buying together, and one of you is contemplating proposing before close of escrow, the tax math is worth a conversation with your CPA before the ring shows up. We have walked this through with clients in our Palo Alto practice and our Mountain View practice.

California Conformity

California conforms to the federal mortgage-interest rules through R&TC §17204, which adopts IRC §163 by reference (subject to California's selective conformity dates and modifications under R&TC §17024.5). For most California residents the $750,000 acquisition-debt cap applies on the state return as well; the state did not adopt a pre-TCJA $1,000,000 cap as a state-only carveout, despite some legislative interest in doing so. Confirm the current conformity date with your CPA, especially if OBBBA 2025 changes flow into a future California conformity bill.

California also conforms to the federal HELOC use-test rule. The "deduct only if used to buy, build, or substantially improve" standard applies on Form 540 the same way it applies on Form 1040.

One California-specific nuance: the state did not conform to the federal $10,000 SALT cap. So California itemizers may find themselves itemizing on the state return even when they take the standard deduction federally. That is worth running both ways at year-end.

Standard Deduction vs Itemizing

The post-TCJA standard deduction is high enough that many homeowners with substantial mortgage interest still don't itemize. For 2026 (projected), the standard deduction is roughly $30,725 MFJ and $15,375 single. Your itemized total has to clear that to make itemizing worthwhile.

For a Bay Area buyer paying $50,000 to $100,000 of deductible mortgage interest plus $10,000 of capped SALT plus some charitable, itemizing is usually the right call. For lower-balance loans or owners further into amortization, the standard deduction often wins. Run both, every year. The IRS does not pay you to itemize a smaller number than the standard.

Practical Steps for Bay Area Homeowners

1. If you are buying: ask your lender for a year-one interest projection. Run the §163 ratio with your CPA before you commit to the loan amount. The cap is part of the deal economics, not an afterthought at tax time.
2. If you have a pre-12/16/2017 loan: guard the grandfathering. A casual refinance can cost you the $1M cap on a meaningful portion of your balance.
3. If you are considering a HELOC or cash-out refinance: document use. Open a separate account, pay qualifying expenses directly from it, save receipts and permits.
4. If you are buying with an unmarried partner: review the Voss structure before close. Title and note liability both matter.
5. If you paid refi points last year: set the amortization schedule in your tax software now. The trickle deduction is easy to lose track of across years and preparers.
6. If you are at the margin between itemizing and the standard deduction: consider bunching (paying two years of property tax, charitable giving, etc. in alternating years) to push above the threshold in one year and take the standard in the other.

What Goes Wrong When Homeowners DIY It

The most common mistake we catch at intake: a Palo Alto homeowner with a $1.8M mortgage who deducted the full Form 1098 interest figure on Schedule A, ignoring the §163 ratio. A return with $130K of mortgage interest reported at face value instead of the cap-limited $54K overstates the deduction by roughly $76K, which at a 37% federal bracket is a $28K overstated refund. That sets up a CP2000 notice 18 to 24 months later with the same dollar amount owed back plus interest and accuracy-related penalties. DIY tax software computes the ratio if you mark the loan as over the cap, but most homeowners never check that box because it is not obvious which box flags the cap. An engagement catches it on intake.

When to Bring in a CPA

The mortgage interest deduction is one of those areas where the rules look simple at the surface and get fact-specific fast. Refinances of grandfathered loans, mixed-use HELOCs, unmarried co-ownership, rental conversions, and second-home arithmetic all reward getting the structure right before the return is filed (and punish getting it wrong, in the form of audit risk or simply lost deductions).

We model these scenarios as part of return prep and proactive planning for clients across San Jose, Sunnyvale, Cupertino, Mountain View, and Palo Alto. If you are buying, refinancing, or sitting on a HELOC and unsure whether the interest is deductible, that is exactly the conversation to have before December 31, not in April.

Schedule a complimentary consultation and we will walk through your specific loan structure, the cap math for your situation, and any planning moves on the table this year.

FAQ

Does the $750,000 cap apply to my second home too?

Yes, on a combined basis. The $750,000 cap is the total acquisition debt across your primary residence and one second home. You don't get $750,000 for each; you allocate one cap across both.

I refinanced my pre-2017 loan in 2023. Did I lose the $1M cap?

Not necessarily. If you refinanced for the same or a lower balance and the same or a shorter term, the $1M grandfathered cap survives, up to the remaining balance of the original loan at the time of refinance. If you cashed out or extended the term, the analysis gets more nuanced. Pull your old and new closing statements and talk to your CPA.

I used a HELOC to pay off credit card debt in 2023. Can I deduct the interest?

No. Under the post-TCJA rules (extended by OBBBA 2025), HELOC interest is deductible only when proceeds are used to buy, build, or substantially improve the home that secures the loan. Credit card payoff is not a qualifying use. The interest is non-deductible regardless of loan name.

My partner and I are buying a $2M home together unmarried. How do we structure it?

Both of you on title (joint tenancy or tenants in common), both of you as co-borrowers on the note. Each of you reports interest paid on your own Schedule A. The 1098 goes to one of you; the other attaches a statement identifying the lender and the primary borrower. Done right, you share $1.5M of capacity rather than the $750K married couples get.

Are mortgage insurance premiums (PMI) deductible?

The PMI deduction was extended several times and has expired and been retroactively reinstated more than once. Check the rule for the specific tax year you are filing; in years it is available, it phases out at higher incomes and most Bay Area earners do not qualify even when the deduction exists.

If I rent out a room in my house, does the mortgage interest treatment change?

Partially. The portion allocable to the rental use is deductible against the rental income on Schedule E rather than as home mortgage interest on Schedule A. The allocation is usually based on square footage or a fair-rooms ratio. Mixed-use properties also affect the §121 home-sale exclusion at sale, so plan the conversion deliberately.