Section 6166 Estate Tax Deferral for Closely-Held Businesses: The 14-Year Installment Election in 2026

Your client built a manufacturing business over forty years. When she dies, the company is worth $40 million — well above the new $15 million federal estate tax exemption that took effect in 2026. The estate owes roughly $10 million in federal estate tax, due nine months after death. But the business has $300,000 in cash. Selling equipment, real estate, or the company itself in nine months would either crush the sale price or end the family's livelihood.

This is the exact problem Section 6166 of the Internal Revenue Code was designed to solve. It lets the executor spread the estate tax attributable to a closely-held business across up to 14 years — five years of interest-only payments followed by ten annual installments of principal plus interest — at a heavily subsidized 2% interest rate on the first slice of deferred tax.

It is one of the most powerful — and most easily forfeited — tools in federal estate planning. Miss the election deadline, fail the 35% test by a hair, or trigger one of the acceleration events buried inside subsection (g), and your client's family is back to a nine-month forced-sale clock.

This guide walks through the mechanics, the 2026 inflation-adjusted numbers, who qualifies, what kills the deferral, and the documentation your records need to support every step.

What Section 6166 Actually Does

Estate tax is normally due nine months after the date of death. Section 6166 lets the executor elect to:

1. Defer payment of the portion of estate tax attributable to a qualifying closely-held business interest
2. Pay interest only for the first four years (the first installment is due on the fifth anniversary of the original due date)
3. Pay the deferred tax in 2 to 10 equal annual installments thereafter
4. Apply a 2% interest rate to the "2-percent portion" of the deferred tax — well below market rates
5. Apply a reduced interest rate (45% of the underpayment rate, or roughly 2.7% in mid-2026) to deferred tax above the 2-percent threshold

The full deferral runs up to 14 years from the original tax due date. In our manufacturing example, instead of writing a $10 million check in nine months, the executor could be paying interest only through 2031 and stretching principal payments out to 2040 — using the business's own cash flow rather than selling assets at fire-sale prices.

That subsidy is enormous. On a 14-year horizon, deferring $10 million at 2-3% blended interest rather than liquidating at distressed prices can preserve millions of dollars of family wealth.

The 35% Threshold: The Eligibility Gate Every Estate Must Clear

The election is only available if the value of the closely-held business interest exceeds 35% of the adjusted gross estate.

"Adjusted gross estate" means the gross estate reduced by deductions allowed under sections 2053 (debts, administration expenses) and 2054 (casualty losses). Charitable and marital deductions under sections 2055 and 2056 are not subtracted.

A simple example. Maria dies with:

• Closely-held S corp stock: $18 million
• Personal residence: $4 million
• Marketable securities: $8 million
• Retirement accounts: $2 million
• Debts and administration expenses: $1 million

Gross estate: $32 million. Adjusted gross estate: $31 million. The business is $18M / $31M = 58%. Maria's estate clears the 35% test.

If Maria had instead held $8M of business stock against the same other assets, the ratio drops to 8/31 = 26% — and the election is unavailable. There is no partial credit.

Aggregating Multiple Businesses

Multiple closely-held business interests can be aggregated to clear the 35% threshold, but only if each interest separately qualifies as a closely-held business under section 6166(b). You cannot combine a 19% stake in one company (which fails the 20% rule, more on that below) with a 30% stake in another to clear the test. Each interest has to pass through the qualifying-business door on its own before the aggregation math even starts.

What Counts as a "Closely-Held Business Interest"

Three structures qualify:

Sole proprietorships. Any interest as a sole proprietor in a trade or business counts. The trade-or-business requirement matters — a portfolio of rental real estate may or may not qualify depending on the level of active management. Pure passive investment is generally excluded.

Partnership interests. Either (a) at least 20% of the capital interest is included in the gross estate, or (b) the partnership has 45 or fewer partners. Family attribution rules apply: a decedent can be treated as owning interests held by certain family members for purposes of the partner-count test.

Corporate stock. Either (a) at least 20% of the voting stock is included in the gross estate, or (b) the corporation has 45 or fewer shareholders.

The 45-partner/shareholder thresholds are particularly important for family businesses with multiple generations of equity owners — a sprawling family LLC can still qualify even if no single member owns 20%.

Passive Assets Are Stripped Out

Here is where many practitioners trip. The value of a closely-held business interest is reduced by the value of any passive assets held by the business. Passive assets include marketable securities, idle cash held in excess of working-capital needs, and stock in unrelated subsidiaries (with limited exceptions for an active subsidiary in which the parent owns at least 20%).

A holding company that owns $5M of operating assets and $15M of marketable securities does not give you a $20M qualifying interest — it gives you $5M. That can swing the 35% test, especially for families that have parked liquid wealth inside the operating entity over the years.

The 2-Percent Portion: The 2026 Math

The famously subsidized 2% interest rate doesn't apply to all of the deferred tax. It applies only to the 2-percent portion, which is calculated as follows:

1. Start with the tentative estate tax on the sum of $1,000,000 (the statutory base) plus the applicable exclusion amount
2. Subtract the unified credit applicable to the estate
3. The result is the maximum deferred tax that qualifies for the 2% rate

The statutory $1,000,000 is inflation-adjusted annually. For 2026, the inflation-adjusted dollar amount is $1,940,000. Combined with the 2026 unified exemption of $15 million per individual (raised by the One Big Beautiful Bill Act), the 2-percent portion typically caps out at roughly $776,000 of deferred tax for a 2026 decedent — calculated as 40% of $1,940,000.

Tax deferred above the 2-percent portion accrues interest at 45% of the regular underpayment rate. With the IRS underpayment rate at 6% in the second quarter of 2026, that translates to roughly 2.7% on the excess. Both rates are recalculated each quarter as the underpayment rate moves.

One important wrinkle: interest paid on deferred Section 6166 tax is not deductible for either income tax (Section 163(k)) or estate tax (Section 2053) purposes. That changed with the 1997 reform. Estates make the deferral choice on after-tax economics — the rate subsidy needs to be worth more than the loss of the interest deduction.

How and When to Make the Election

The election is made on a timely-filed Form 706, the federal estate tax return. Specifically:

• Check the box on Form 706, Part 3, Line 3
• Attach a notice of election that includes the specific information required by Treasury Regulation § 20.6166-1(b): the decedent's name and TIN, the amount of tax to be paid in installments, the date of the first installment, the number of installments, the property qualifying for the election, the value of the gross estate, and the value of the closely-held business interest

The Form 706 must be filed by the regular due date — nine months after the date of death, plus any extension granted. Critically, there is almost no available 9100 relief for a late-filed Section 6166 election when the return itself is late. The election essentially lives or dies with timely return filing. Treat the nine-month clock as immovable.

The Protective Election

If you are not sure the estate will clear the 35% test — perhaps because asset valuations are still in flight or an audit may move numbers — file a protective election. A protective election locks in the right to defer if the final values (after exam, appeals, or Tax Court) end up satisfying the requirements. It is essentially free insurance and should be standard practice for any estate where the qualifying business interest is anywhere near the 35% line.

Post-Assessment Election for Deficiencies

If the original return passed without electing and a later examination produces a deficiency attributable to the closely-held business, the executor has a 60-day window to elect Section 6166 treatment for the deficiency portion only. This is a narrow safety valve, not a substitute for getting the original election right.

Acceleration Events: What Kills the Deferral

Three categories of events can terminate the deferral and accelerate the unpaid tax. Every executor managing a 6166 estate needs to understand these, and the estate's accountant needs ongoing visibility into the underlying business activity.

1. The 50% Disposition Rule

If the cumulative dispositions, sales, exchanges, or withdrawals of money or property attributable to the closely-held business interest equal or exceed 50% of the value of the interest (measured at date of death), the deferral terminates. The entire unpaid tax becomes due on notice and demand.

This is the most common trigger and the most dangerous. Watch for:

• Owner distributions in excess of compensation
• Liquidating distributions to one branch of the family
• Sale of a division
• Capital withdrawals masquerading as loans
• Redemption of stock by the estate or heirs

A Section 303 stock redemption — which lets the estate redeem stock without dividend treatment to fund estate tax and administration costs — is specifically excepted from the 50% test. That carve-out is one reason Sections 303 and 6166 are often planned together.

2. Undistributed Net Income (After Year 4)

For taxable years of the estate ending on or after the due date of the first installment (year 5 onward), any "undistributed net income" of the estate must be applied to prepay the deferred tax. Undistributed net income is roughly the estate's distributable net income, less amounts actually distributed and less the estate's own income tax.

In practice, this means an estate that is sitting on substantial income — say, dividends or interest accumulating in the estate's accounts after year 4 — cannot simply stockpile cash while paying only the minimum installment. The IRS effectively requires that excess income flow through to reduce the deferred balance.

3. Late Payment

A missed installment of principal or interest will accelerate the entire deferred balance unless the executor catches up within six months of the due date. The grace period exists, but it comes with a penalty and is not a routine planning tool.

The Special Lien Under Section 6324A

The IRS has the right to demand security for the deferred tax — historically by requiring a surety bond. Bonds for a 14-year horizon are expensive and sometimes unavailable. The alternative, available since 1981, is the Section 6324A special estate tax lien.

The executor and all parties with an interest in the qualifying property sign a lien agreement designating "lien property" of sufficient value to cover the deferred tax plus the four years of required interest. Once recorded, the lien:

• Substitutes for the bond requirement under sections 2204 and 6165
• Replaces the general estate tax lien under section 6324 with respect to the lien property
• Provides a Section 2204 discharge of the executor's personal liability for the deferred tax

After the Tax Court's decision in Estate of Roski v. Commissioner, the IRS may not automatically require a bond or 6324A lien in every 6166 case. Security is supposed to be determined case-by-case based on whether the government's collection interest is at risk. In practice, many estates still end up posting a 6324A lien voluntarily because it provides the personal-liability discharge.

Practical Bookkeeping for a 6166 Estate

A Section 6166 estate is not a one-and-done filing. It is a 14-year compliance commitment that needs clean financial records every year. Among the critical items:

• Track the 50% disposition pool. Every sale, distribution, or withdrawal of business property needs to be logged against the date-of-death value of the qualifying interest. A simple running total prevents nasty surprises in year nine.
• Reconcile estate income annually. Once you hit year 5, distributable net income, distributions, and estate income tax all feed into the undistributed-net-income calculation. Errors here can trigger acceleration.
• Maintain valuations of lien property. Lien property values can drift. The IRS can demand additional property if values fall below required coverage.
• Document the qualifying interest at death. Detailed appraisal support, including any passive-asset stripping, will be the foundation for every future calculation.
• Calendar interest-only installments. The first four years are interest only; missing any of them is an acceleration event after the six-month grace period.

This is exactly the kind of long-running, audit-sensitive record set where plain-text accounting earns its keep. Estate records that need to survive 14 years of installments — and potentially an IRS examination years after they are created — should be transparent, version-controlled, and trivially reviewable.

When Section 6166 Is the Right Answer — And When It Isn't

Section 6166 is the right tool when:

• The estate genuinely needs liquidity time. Forced sale of the business or its core assets would destroy value.
• The business produces enough ongoing cash flow to service interest and amortize principal over the deferral period.
• The family wants to keep operating the business and has succession in place.
• You have priced the post-1997 loss of interest deductibility into the analysis and it still pencils out.

It is the wrong tool when:

• The estate has ample liquid assets and Section 6166 is being used reflexively. The rate subsidy is real, but the 14-year compliance burden, lien, and acceleration risk have real costs too.
• The family plans to sell the business within the deferral window. Once cumulative dispositions hit 50%, the whole deferral collapses.
• The qualifying interest is a thin pass — barely 35% — and asset values could move on audit. Without a protective election, the risk is binary.
• The estate's other income would routinely trigger the undistributed-net-income acceleration rules from year 5 onward.

A common alternative or supplement is Section 6161 — a discretionary extension of time to pay estate tax for reasonable cause, available for up to 10 years. Section 6161 is easier to qualify for, but the interest is full underpayment rate, there is no 2% slice, and it depends on IRS discretion rather than statutory right. The two are not mutually exclusive: an estate can use 6161 for non-business tax and 6166 for the business slice.

A 14-Year Election Demands 14 Years of Discipline

The estates that succeed at Section 6166 share one trait. Long before death, they put accounting and reporting systems in place that can support a multi-decade installment schedule — clean separation of business and personal assets, defensible valuations, transparent income flows, and records that an executor (and an IRS examiner) can audit without forensic effort.

The estates that fail at Section 6166 usually fail because of bookkeeping decisions made years before the election. Passive assets parked inside the operating entity. Owner draws and capital withdrawals reclassified as loans without documentation. Family-member transactions never papered. Each of these can either break the 35% test or, after the election, trigger one of the acceleration events in subsection (g).

Keep Your Financial Records Audit-Ready for the Long Haul

Whether you are administering a 6166 estate today, planning one a decade out, or just running the business that will eventually be part of one, the foundation is the same: financial records that are accurate, transparent, and durable across the people and software generations that will touch them. Beancount.io provides plain-text, version-controlled accounting that fits this use case naturally — every transaction is a readable text record, every change is tracked, and your data is portable rather than locked inside a vendor's database. Get started for free and see why developers, finance professionals, and family-business advisors are switching to plain-text accounting.