The $750K Mortgage Interest Deduction Cap for HNW

The 2017 Tax Cuts and Jobs Act (TCJA) made one change to mortgage interest deductibility that quietly reshaped HNW home-financing math: it lowered the cap on deductible acquisition indebtedness from $1,000,000 to $750,000 for loans originated after December 15, 2017.

For most middle-income borrowers, the change was invisible — their mortgages were already below the new cap. For HNW buyers carrying $1.5–4M jumbos, the change took the after-tax cost of a mortgage and pushed it materially higher than most borrowers realize.

This post walks through exactly how the cap works, what the math looks like at different mortgage sizes, and how to think about it relative to alternatives like SBLOCs and HELOCs. For the broader picture — how the deduction cap reshapes the choice between jumbo, SBLOC, and asset-depletion underwriting — see our complete guide to high net worth home buyer financing.

The Mechanics of the Cap

Under IRC Section 163(h)(3), interest on "acquisition indebtedness" — debt incurred to buy, build, or substantially improve a qualified residence and secured by that residence — is deductible up to a cap on the principal balance of the debt.

For loans originated after December 15, 2017: the cap is $750,000 of acquisition indebtedness across the taxpayer's primary and qualifying second residence combined.

For loans originated on or before December 15, 2017 (or refinances of such loans, in some cases): the cap is $1,000,000.

A taxpayer can deduct interest on the deductible portion only. If your mortgage balance is $1,500,000, you can deduct interest on the first $750,000 and not on the remaining $750,000.

The mechanics of the calculation:

1. Determine the deductible portion: minimum of mortgage balance or $750K cap.
2. Calculate the deductible fraction: deductible portion / total mortgage balance.
3. Apply the deductible fraction to interest paid.
4. Multiply by your marginal federal tax rate to get the federal benefit.
5. Add state benefit if your state allows mortgage deduction.

What This Looks Like Across Mortgage Sizes

Assume a 30-year jumbo at 6.75% interest, 37% federal bracket, 9% state (a high-tax-state HNW profile), and the state allows the mortgage deduction up to the federal cap.

Mortgage Size	Year 1 Interest	Deductible Fraction	Year 1 Federal Benefit	Year 1 State Benefit	After-Tax Effective Rate
$750,000	$50,625	100%	$18,731	$4,556	3.65%
$1,000,000	$67,500	75%	$18,731	$4,556	4.32%
$1,500,000	$101,250	50%	$18,731	$4,556	5.20%
$2,000,000	$135,000	37.5%	$18,731	$4,556	5.59%
$3,000,000	$202,500	25%	$18,731	$4,556	5.97%

The pattern is striking. At $750K, the after-tax effective rate is 3.65% — barely above prime in real terms. At $3M, the effective rate climbs to nearly 6%, almost matching the SBLOC rate at most venues.

The "mortgage is cheap" intuition that HNW borrowers carry into a home purchase comes from the world where the deduction applied to most of the loan. Above the cap, the marginal cost of additional mortgage debt is pretax — and at 6.75%, that is a meaningful number.

Why This Reshapes the SBLOC Comparison

The standard analysis assumes a fully-deductible mortgage. With that assumption, a 6.75% jumbo at a 37% bracket has an after-tax cost of roughly 4.25% — clearly cheaper than a 6.25% non-deductible SBLOC.

Run the analysis with the actual deductibility math on a $2M jumbo and the picture shifts. The blended after-tax rate is about 5.59% — barely cheaper than the 6.25% SBLOC, and the SBLOC has zero capital gains drag from any down payment liquidation.

For a buyer above $1.5M of mortgage debt, the marginal cost of borrowing more in jumbo form is closer to the SBLOC rate than the headline mortgage advantage suggests. This is the math that makes hybrid structures — jumbo up to $750K plus SBLOC for the rest — compelling on the right facts.

The Hybrid Structure That Captures the Cap

The optimal structure for a $2M+ home purchase with a sizable taxable portfolio often looks like:

1. Take a jumbo mortgage of exactly $750,000. Every dollar of interest is deductible.
2. Fund the remainder of the home from a combination of cash savings and SBLOC drawn against the portfolio.
3. The jumbo provides cheap, fully-deductible permanent financing for the maximum tax-advantaged size.
4. The SBLOC provides flexible, non-amortizing financing for the portion above the deduction cap.

Worked example: $2M home purchase by a buyer with $300K cash savings.

• Down payment from cash: $300K.
• Jumbo mortgage: $750K (fully deductible, sized to the cap).
• SBLOC draw: $950K.

Annual interest cost:

• Mortgage at 6.75% on $750K = $50,625 (100% deductible at 46% combined = $23,287 federal+state benefit; net cost $27,338).
• SBLOC at 6.25% on $950K = $59,375 (not deductible; net cost $59,375).

Total annual after-tax interest: $86,713 on $1.7M of debt. Effective blended rate: 5.10%.

Compare to a single $1.7M jumbo at 6.75%:

• Annual interest: $114,750.
• Deductible portion: $750K / $1.7M = 44.1% of $114,750 = $50,604.
• Tax benefit at 46% combined: $23,278.
• Net cost: $91,472. Effective rate: 5.38%.

The hybrid saves about $4,759/year in after-tax cost, or roughly $48K over a 10-year hold. Not transformative, but real money for a small structural change.

State Rules That Modify the Cap

The federal cap is $750K, but several states use different limits:

• California: allows mortgage deduction on up to $1,000,000 of acquisition indebtedness. Higher than federal. Improves the after-tax cost on $750K–$1M jumbos.
• New York: generally conforms to federal but with its own AMT-like adjustments. Effective treatment can be more restrictive.
• Texas, Florida, Washington, Tennessee, Nevada: no state income tax, so the federal cap is the only one that matters. After-tax cost on big jumbos is correspondingly higher because there is no state benefit on the deductible portion.

For a borrower in California with an $850K mortgage, the federal deduction applies to the first $750K and the California deduction applies to all $850K. The state-only benefit on the $100K above the federal cap adds about $675 of tax savings per year.

These adjustments are small relative to the federal benefit but worth modeling correctly.

The Refinance Trap

A specific situation that catches HNW borrowers: you have an old $1.0M mortgage originated before December 15, 2017, with $1M of deductible indebtedness under the pre-TCJA cap. You refinance to take advantage of lower rates.

If the refinance amount equals or is less than the existing balance, the old $1M cap is grandfathered — you keep the higher deduction limit on the new loan.

If the refinance amount exceeds the old balance — even by a dollar — the loan is treated as a new origination subject to the $750K cap, and you lose the $250K of additional deductibility on the existing balance.

This grandfathering rule is a real planning consideration. A homeowner with a grandfathered $950K mortgage should think carefully before refinancing for cash-out at $1.1M; the loss of grandfathered status can be more expensive than the rate savings.

The Charitable Deduction Interaction

For high-income HNW borrowers, the mortgage interest deduction is one of the larger itemized deductions on the return. It interacts with the SALT cap ($10K), charitable deductions, and other Schedule A items.

A specific consideration: the standard deduction in 2026 is roughly $30,000 for married filing jointly. To get any benefit from the mortgage interest deduction, your total itemized deductions must exceed the standard deduction. A buyer with a small mortgage, no charitable giving, and no other itemized deductions may not benefit from the mortgage interest deduction at all because they are already taking the standard deduction.

For HNW buyers with significant SALT (capped), charitable giving, and mortgage interest, itemizing is virtually always the better choice. But the marginal value of each dollar of mortgage interest is only the federal rate (37%) plus the state rate, applied to interest above the standard deduction threshold. Run the full Schedule A in any scenario analysis.

How This Changes the Decision Framework

The pre-TCJA assumption was: "Mortgage interest is fully deductible at marginal rate. Get the biggest mortgage you can comfortably service and use the cheap money." This is no longer the right heuristic for HNW buyers.

The post-TCJA framework should be:

1. Size the mortgage to the $750K cap when possible — that is the cheapest dollar of debt you can carry.
2. For amounts above $750K, evaluate whether the marginal cost of additional jumbo (pretax) is cheaper than alternatives (SBLOC, HELOC, cash from portfolio).
3. Consider hybrid structures that combine a $750K-sized jumbo with another financing source for the excess.
4. Run the math at your actual marginal rate, including state, and against the standard deduction baseline.
5. Account for the cap when planning future home purchases or refinances — the limit applies across all qualifying residences combined.

This framework looks more complicated than the pre-TCJA version, but it produces materially better outcomes on real HNW math.

Model the Cap Correctly

Most home calculators ignore the deduction cap entirely. They either assume 100% deductibility (which overstates the mortgage advantage on big loans) or 0% deductibility (which overstates SBLOC competitiveness).

Stockstead applies the cap correctly out of the box. Mortgage interest above $750K is treated as non-deductible; below the cap, it gets the full federal-plus-state benefit. The math reflects what your return will actually look like.

Compare Your Options with Stockstead →

The $750K cap is the most underappreciated change in HNW home finance. Models that ignore it will keep producing the wrong answer.