IRC 6166: Estate Tax Deferral for Closely Held Businesses

The IRC 6166 estate tax deferral lets an executor pay the federal estate tax attributable to a qualifying closely held business over as many as 14 years instead of within nine months of death. It does not reduce the tax. It restructures the timeline, so the business can generate cash to cover the bill rather than being sold under pressure. To use it, the business interest must clear specific ownership tests and make up more than 35% of the decedent’s adjusted gross estate, and the executor must elect the deferral on a timely filed Form 706.

Which Business Interests Qualify

The threshold question is whether the decedent’s interest was in an active trade or business. Section 6166 draws a hard line between operating businesses and entities that passively hold assets. A company with employees delivering services, manufacturing products, or running day-to-day operations clears the bar. A holding entity collecting rent on triple-net leases, holding marketable securities, or sitting on undeveloped land almost certainly does not.

Three legal structures can qualify: a sole proprietorship, a partnership interest, or corporate stock. Each must satisfy the active-business standard and at least one of two ownership tests that define “closely held.”¹

The 20% ownership test. The decedent’s gross estate includes at least 20% of the partnership’s total capital interest, or 20% or more in value of the corporation’s voting stock.
The 45-or-fewer-owners test. The partnership has 45 or fewer partners, or the corporation has 45 or fewer shareholders.

Either test is enough. A sole proprietorship automatically satisfies both.

Interests held by the decedent’s spouse, children, grandchildren, and parents are attributed to the decedent for purposes of the ownership tests.¹ Attribution can push an estate over the 20% line when the decedent’s personal holdings alone would not.

The 35% of Adjusted Gross Estate Threshold

Once the business qualifies as closely held, the estate must show that the interest’s value exceeds 35% of the decedent’s adjusted gross estate. The adjusted gross estate is the total gross estate minus the deductions allowed under IRC Sections 2053 and 2054 for debts, administration expenses, funeral costs, and casualty losses.² Values are those used for estate tax purposes, either at date of death or at the alternate valuation date.

Small swings can wreck this ratio. The numerator is the net value of the qualifying business interest after subtracting passive assets and any debt secured by business property. The denominator is the adjusted gross estate. If executor fees run higher than estimated, or a valuation nudges the business interest down, the estate can lose access to the entire deferral.

Passive Assets Come Out

Even when a business qualifies as active, the value of any passive assets it holds is stripped out for 6166 purposes. Passive assets include excess cash in investment accounts, rental properties unrelated to operations, and stock in another company.¹

Stock in a subsidiary is automatically passive unless the parent owns at least 20% of the subsidiary’s voting stock (or the subsidiary has 45 or fewer shareholders) and at least 80% of each company’s assets are used in active operations. When both conditions are met, parent and subsidiary are treated as a single corporation and the subsidiary stock is not passive.

A business worth $10 million on paper might hold $3 million in passive investments, leaving $7 million eligible. That reduced figure is what counts toward the 35% threshold and drives how much tax can actually be deferred.

Combining Multiple Businesses

An estate holding interests in several businesses, none of which individually clears 35%, can combine them if the gross estate includes at least 20% of the total value of each business being combined.¹ Family attribution applies when measuring that 20%. A business in which the estate holds less than 20% cannot be added to the pool, even if the combined total would clear 35%.

Making the Election

The executor makes the election by attaching a notice to a timely filed Form 706. There is no dedicated IRS form. The notice must identify the decedent, state the value of the gross estate and the closely held business interest, specify the amount of tax to be deferred, and indicate the number of installments elected, between two and ten.¹ The deadline is the Form 706 due date including extensions: nine months after death, plus any six-month extension.³

Miss that deadline and the election is gone. The IRS enforces it strictly.

Only the portion of estate tax attributable to the closely held business is deferred. If the business represents 40% of the adjusted gross estate, 40% of the net estate tax is eligible for installments; the other 60% is due on the original schedule.

Protective Elections

When qualification is uncertain, usually because a valuation is in dispute, a protective election preserves the right to defer if the final numbers work out. The executor files the protective election with the timely Form 706, stating that the election is contingent on final determined values meeting the 35% threshold. If a later audit adjusts values in a way that confirms qualification, the executor perfects the election by notifying the IRS of the final tax liability and providing all required details within 60 days of the final value determination. Without a timely protective election on file, an estate that only qualifies after audit loses the deferral.

Payment Schedule and Interest Rates

The deferral runs in two phases. For up to five years after the original estate tax due date, the estate pays interest only, no principal. The executor picks when principal payments begin, any date up to that five-year mark.¹ Once principal begins, equal annual installments run for up to ten years. At the outside, that is five years of interest only followed by ten years of principal and interest: a 14-year window.

Interest is split. A special 2% fixed rate applies to the “2-percent portion” of the deferred tax, calculated on a base of an inflation-adjusted $1 million (the figure was $1,900,000 for 2025) plus the applicable exclusion amount, reduced by the applicable credit.⁴⁵ Deferred tax above the 2% portion carries interest at 45% of the standard IRS underpayment rate, which fluctuates quarterly.⁶ If the underpayment rate is 8%, the estate pays 3.6% on that upper slice.

Interest accrues annually and must be paid each year, including during the five-year principal holiday. Neither the 2% interest nor the reduced-rate interest is deductible for income tax purposes.

What Can Accelerate the Balance

The deferral is a privilege, and several events can cause the full unpaid balance to come due at once.

Selling or Withdrawing Business Assets

If the combined value of dispositions (sales, exchanges, distributions) and withdrawals of money or property from the business reaches 50% or more of the qualifying interest’s value, the deferral terminates and the remaining balance is due on demand.¹ The 50% figure is measured against the value that originally qualified, not current fair market value.

Several exceptions protect normal administration. Distributing the business interest to someone entitled to it under the will or applicable inheritance law is not a disposition, and that treatment carries through subsequent family transfers. Redemptions of stock to pay estate taxes and expenses under IRC 303 are excluded from the disposition count, though the qualifying interest is reduced by the redeemed stock’s value and the estate must pay estate tax at least equal to the redemption proceeds by the next installment due date. Stock exchanges in certain tax-free reorganizations and spinoffs do not trigger acceleration, and the stock received is treated as qualifying interest going forward.

Missing a Payment

A missed principal or interest payment triggers acceleration of the entire unpaid balance. The estate has a six-month cure window, but curing costs the favorable interest rates on the late amount and adds a penalty of 5% of the overdue payment for each month or partial month it is late.¹ If nothing is paid within six months, the full remaining balance accelerates.

Undistributed Net Income

Starting with the tax year ending on or after the first principal installment due date, the estate must apply its undistributed net income toward the deferred balance. Undistributed net income is the estate’s distributable net income under IRC 643 minus amounts actually distributed to beneficiaries, income tax paid by the estate, and estate tax paid that year.¹ The payment is due by the income tax filing deadline for that year. Failing to pay it over is treated as a missed installment.

Securing the Deferred Tax

The IRS wants assurance that a 14-year payment stream will actually be collected. The executor has two routes. The first is a special lien under IRC 6324A, which attaches to property expected to satisfy the deferred tax plus projected interest and takes the place of a surety bond.⁷ Everyone with an interest in the liened property must sign the agreement. It encumbers the property but avoids ongoing premiums.

The alternative is a surety bond. Bond premiums typically run in the low single digits of the deferred amount per year, with rates varying by risk profile and bonding company. On large deferred balances, cumulative bond costs can materially erode the benefit of the deferral. Failing to provide adequate security when the IRS demands it can end the installment privilege.

If the IRS Denies the Election

When the IRS denies an election, or determines that a valid election has ceased to apply, it issues a formal determination letter: Letter 3571 for an outright denial, or Letter 3570 for termination of a previously valid election. The estate must exhaust administrative remedies within the IRS, including a conference with IRS Appeals after a preliminary determination.⁸

If administrative appeals fail, or if the IRS does not act on a determination request within 180 days, the estate can petition the U.S. Tax Court for a declaratory judgment under IRC 7479. The Tax Court can rule on whether the initial election was valid and whether an existing election should continue. Administrative remedies are considered exhausted after 180 days of IRS inaction if the estate took all reasonable steps during that period to obtain a determination.⁸

The stakes at that stage are large. An estate that loses its 6166 election faces immediate payment of a bill that may run into the millions, and the business may not survive the demand. A protective election filed at the outset, careful records showing the business’s active operations, and prompt responses to IRS inquiries are the practical defenses against ever getting there.

1
Office of the Law Revision Counsel. 26 US Code 6166 – Extension of Time for Payment of Estate Tax Where Estate Consists Largely of Interest in Closely Held Business
2
Legal Information Institute. 26 US Code 6166(b)(6) – Adjusted Gross Estate Definition
3
Internal Revenue Service. Filing Estate and Gift Tax Returns
4
Office of the Law Revision Counsel. 26 USC 6601 – Interest on Underpayment, Nonpayment, or Extensions of Time for Payment, of Tax
5
Internal Revenue Service. Revenue Procedure 2024-40
6
Internal Revenue Service. Revenue Procedure 98-15
7
Office of the Law Revision Counsel. 26 US Code 6324A – Special Lien for Estate Tax Deferred Under Section 6166
8
Internal Revenue Service. Revenue Procedure 2005-33