WTF is this?
Borrow, don't sell: the basic idea
April 13, 2026
TL;DR: Borrowing against a stock portfolio is structurally similar to a home equity loan — the asset stays in place, generates returns, and serves as collateral for the loan. The main obstacle has always been the rate: banks charge 8-13% on portfolio-backed loans, which is too high for the math to work. A box-spread approach closes most of that gap by using the options market instead of a bank, getting to roughly 4% before the tax benefit and closer to 3% after.
The 2-minute version
Most people have heard of a HELOC — a home equity line of credit. The house serves as collateral. The bank lends against it at a rate tied to prime. The homeowner gets cash without selling the house.
Portfolio-backed borrowing works the same way, with one key difference: the collateral is a brokerage account instead of a house.
The appeal is the same. The stock portfolio keeps growing. No capital gains tax is triggered by the borrowing itself. The investor gets cash when they need it without permanently giving up the asset.
Consider Sarah. She’s a self-employed consultant who has been steadily building a taxable brokerage account for fifteen years. She has $1.5 million in index funds. Her current mortgage is paid off. Now she needs $300,000 for a combination of home renovation and a family expense, and she doesn’t want to sell assets that are sitting at a significant gain.
One option: a HELOC on the house. The house is worth $2 million, mortgage is zero — she qualifies easily. The rate is prime plus about 1%, which runs around 8-9% in 2026. She’d pay roughly $24,000-$27,000 in interest per year, and that interest is only deductible against mortgage interest, which is capped at $750,000 in debt.
Another option: borrow against the brokerage at 4%. A synthetic loan on her $1.5 million portfolio can release $300,000 at a fixed rate around 3.70%. The interest runs around $11,100 per year, less than half the HELOC cost. And the interest shows up as a capital loss deduction with no restriction on how the proceeds were used.
The difference isn’t small. Over three years: HELOC at 8.5% = roughly $76,500 in interest. Synthetic loan at 3.70% fixed = roughly $33,300. Sarah keeps $43,000 more. Plus she has the tax deduction.
The nerdy version
How collateralized lending works
When a lender accepts collateral, they take the asset’s value as security for the loan. If the borrower defaults, the lender sells the collateral to recover the principal. The tighter the collateral, the lower the rate — because the lender’s risk is lower.
A house is good collateral. It has a physical existence, it’s relatively stable in value, and it’s illiquid enough that the borrower won’t disappear with it. Mortgage rates are low because houses are excellent collateral.
A diversified stock portfolio is also good collateral — arguably better than a house in some respects. A well-diversified S&P 500 portfolio has never gone to zero. It’s instantly liquid. In a margin call, the lender doesn’t need to wait for a foreclosure sale; the assets can be liquidated in minutes.
Yet banks charge far more for portfolio-backed loans than for mortgages. A few reasons: retail margin lending involves more regulatory complexity; banks fund margin loans from their own balance sheets with credit-rating implications; and there’s less competition in the space. The rate premium over mortgages is partly structural and partly friction.
The two kinds of portfolio-backed borrowing
Securities-backed lines of credit (SBLOCs), also called Pledged Asset Lines, are bank products. The borrower pledges a portfolio, the bank extends a line, typically at 50-70% of the portfolio’s value. Schwab charges 6-8% on their Pledged Asset Line. Fidelity’s margin rates start at 10.575% and step down to around 7.5% for balances over $1 million. The interest is only tax-deductible if the proceeds fund additional investments.
Box-spread loans — the approach this site explains — use the options market rather than a bank. The borrower (or their adviser) opens a position on the S&P 500 options market. The position creates a cash inflow. At expiration (anywhere from 3 months to 5 years), the position unwinds and the principal plus interest flows out. The implied rate tracks Treasury yields closely, currently around 3.70-3.95%. The interest cost appears as an annual capital loss, deductible regardless of how the proceeds are used.
Rate comparison (April 2026):
| Product | Approximate rate | Tax-deductible? |
|---|---|---|
| Box spread | ~3.70–3.95% | Yes, always (capital loss) |
| 30-year jumbo mortgage | ~6.47% | Yes, up to $750K of debt |
| Schwab PAL (SBLOC) | 6-8% | Only if used to invest |
| Fidelity margin | 7.5-10.6% | Only if used to invest |
| Schwab margin | 8-13% | Only if used to invest |
Rates change; verify before acting.
The balloon payment structure
Most box-spread loans — particularly fixed-rate ones — are structured like zero-coupon bonds. No payments are made during the loan term. The full principal plus all accrued interest is due at maturity. This is different from a mortgage or HELOC, which require monthly payments.
That structure requires planning. Sarah doesn’t make monthly interest payments on her 3-year fixed loan. Instead, she needs to have $300,000 + $33,300 = $333,300 available at the 3-year mark. Common approaches: set aside monthly contributions into a Treasury or money-market account that will cover the balloon; ladder the loan into tranches with staggered expirations (e.g., $100,000 due each year for three years); or roll the principal at maturity (refinance) and pay only the interest each cycle.
Floating-rate loans behave differently — they function more like an open line of credit with no fixed maturity, but periodic interest can (and usually should) be paid to keep the balance manageable.
How collateral requirements work in practice
A $1.5 million portfolio supporting a $300,000 loan is a 20% loan-to-value ratio. That’s very conservative. The portfolio would need to fall more than 80% before the margin math became an issue at that level.
At higher LTVs the math is tighter. At 50% LTV (borrowing $750,000 against a $1.5 million portfolio), a 28.57% decline in the portfolio value would trigger a margin call. The sequence would be: broker notifies the borrower; the borrower has a window to deposit cash or sell other assets; if they don’t, the broker liquidates enough of the portfolio to restore the required margin.
The practical risk isn’t theoretical — it’s concentrated portfolios and timing. A broadly diversified portfolio falling 29% is uncommon but not impossible (2008-2009 saw the S&P 500 fall roughly 55% peak to trough over 18 months). A single-stock portfolio falling 29% in a week is unremarkable.
Advisers who manage these loans often recommend keeping a HELOC or other liquid backstop available in case a temporary market decline triggers a call at an inconvenient time.
What this isn’t
This is not the same as keeping a margin balance for speculation. Traditional margin accounts are used to buy more securities on leverage — amplifying both gains and losses. A portfolio-backed loan here is borrowing against existing securities to fund something else (a house, a renovation, a tax bill). The portfolio itself remains unchanged.
This does not eliminate the need to repay the loan. The loan is real debt. If the portfolio grows slower than expected and the borrower also can’t service the balloon, the outcome is a margin call or a forced sale. This works best when the borrower has a credible plan to repay.
The rate advantage shrinks without a tax benefit. Someone in a low tax bracket with no capital gains to offset doesn’t capture the full benefit of the Section 1256 deduction. They still get a better rate than a bank margin loan, but the advantage is smaller. See the tax series for the full breakdown.
If you want to actually do this
The next article in this series explains who specifically should consider this approach and who should probably skip it: “Who should (and shouldn’t) use this”.
The mechanics of how the position actually works — what four legs are opened, what happens at expiration — are in the options series, starting with “The four-leg structure”.
When ready to talk to a firm that executes portfolio-backed borrowing, request a referral. I’ll send a referral code and an intro. Talk to your CPA first. See full disclosure.
Sources
- IRS Publication 550 — Investment Income and Expenses; covers investment interest deduction limitations
- IRS Form 4952 — Investment Interest Expense Deduction; covers the restriction on SBLOC deductibility
- Cboe SPX Options Specifications — SPX contract specs
- Schwab Pledged Asset Line Rates — Current rates
- Fidelity Margin Rates — Current rates
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