What Is Substantial Authority in Tax Law: Sources and Penalty Relief

Substantial authority in tax law is an objective standard the IRS uses to decide whether the legal support behind a position on your return is strong enough to shield you from the 20 percent accuracy-related penalty. It sits in the middle of the confidence spectrum: tougher than “reasonable basis,” easier than “more likely than not,” which requires a greater than 50 percent chance of being upheld.¹ Practitioners often describe it as roughly a 40 percent likelihood of success, though no regulation fixes an exact number.

The standard matters when a position is aggressive enough that, if the IRS challenges it, the resulting underpayment could carry a penalty on top of the extra tax.²

How the Standard Is Measured

What you believed about your position doesn’t enter the analysis. Neither does the odds of being audited. The question is whether the weight of recognized legal authorities supporting your treatment significantly outweighs the weight of authorities pointing the other way.¹

Timing is more flexible than most people assume. A position qualifies if substantial authority existed either on the date the return was filed or on the last day of the tax year the return covers.¹ A court decision or IRS ruling published after your tax year ended but before you filed can retroactively support what you did.

Which Sources Count as Authority

The regulations give a closed list. Only these sources count:

The Internal Revenue Code and other statutes.
Final, temporary, and proposed Treasury regulations.
Revenue rulings, revenue procedures, notices, announcements, information releases, and other pronouncements published in the Internal Revenue Bulletin.
Decisions of the Supreme Court, Courts of Appeals, District Courts, the Tax Court, and the Court of Federal Claims.
Committee reports, conference committee explanations, and pre-enactment floor statements by a bill’s managers.
The Blue Book, meaning General Explanations of tax legislation prepared by the Joint Committee on Taxation.
Private letter rulings and technical advice memoranda issued after October 31, 1976.
Actions on decisions and general counsel memoranda issued after March 12, 1981.
Tax treaties, their implementing regulations, and official Treasury explanations of them.

That list is exclusive.¹ Conclusions in treatises, legal periodicals, and opinions from tax professionals are explicitly excluded. A detailed memo from your CPA or a law review article analyzing your exact situation cannot, on its own, create substantial authority. Those materials can point you to sources that do count, but they add no independent weight.

How the Weighing Works

The analysis is qualitative. Counting authorities gets you nowhere; a long list of loosely relevant rulings can lose to one well-reasoned Tax Court decision squarely on point. What matters is how closely each authority’s facts match yours, how persuasive its reasoning is, and how directly it applies the relevant law to those facts.

Age matters too. An older ruling can lose force if the underlying statute has been amended or if newer authorities have shifted the analysis. Contrary authorities must be factored in — the exercise mirrors how a court would look at the question, weighing favorable and unfavorable sources together before reaching a conclusion.¹ Even a strong authority weakens if your facts differ materially from the ones it addressed.

The Penalty You Are Trying to Avoid

Meeting the standard blocks the accuracy-related penalty under Section 6662, which is 20 percent of the underpayment tied to a substantial understatement of income tax.² The thresholds for a “substantial” understatement differ by taxpayer type:

For individuals, the understatement must exceed the greater of $5,000 or 10 percent of the tax required to be shown on the return.²
For C corporations, the understatement must exceed the lesser of 10 percent of the tax required to be shown (or $10,000, whichever is greater) or $10,000,000. S corporations and personal holding companies use the individual threshold.²

When substantial authority backs a position, the item is treated as if it were properly reported and drops out of the penalty calculation.² You still owe the additional tax if the IRS wins on the merits. You just avoid the extra 20 percent.

Where the Shield Does Not Reach

For items attributable to a tax shelter, substantial authority alone will not reduce the understatement. The statute blocks that relief.² A tax shelter here means any partnership, entity, investment plan, or arrangement where a significant purpose is avoiding or evading federal income tax.

An individual with a tax shelter position can still escape the penalty, but only by meeting a tougher two-part test: substantial authority for the treatment plus a reasonable belief at the time of filing that the position would more likely than not be sustained on its merits.¹

Reportable transactions have their own regime under Section 6662A. The baseline penalty is 20 percent and rises to 30 percent for listed transactions and other avoidance transactions when disclosure requirements aren’t met.³ Substantial authority is not a recognized defense against a Section 6662A penalty.

Fallbacks When You Cannot Meet the Standard

Adequate Disclosure

For non-tax-shelter positions, disclosure is the standard backup. Attach Form 8275 to your return, describe the position properly, and the substantial understatement penalty can be avoided as long as the position has at least a reasonable basis, the lowest recognized standard.⁴ You trade secrecy for a lower legal bar.

Positions taken contrary to a Treasury regulation require Form 8275-R instead. You must identify the specific regulation you’re challenging, explain why you believe it’s invalid, and show the position represents a good-faith challenge with a reasonable basis.⁵ Neither form helps with tax shelter items.

Reasonable Cause and Good Faith

When both substantial authority and disclosure fall short, one defense remains. No accuracy-related penalty applies if you can show reasonable cause for the underpayment and that you acted in good faith.⁶

Reliance on professional advice is the most common route. Courts look at whether the advisor was competent in the relevant area of tax law, whether you gave the advisor all necessary and accurate information, and whether you actually relied on the advice. Reliance has to be objectively reasonable. Handing an accountant incomplete records and pocketing a favorable answer won’t do it, and the defense is confined to technical tax questions — it won’t rescue you if the underlying facts were misrepresented or if the advisor’s opinion rested on assumptions you knew were wrong.⁷

A Note on Your Preparer

Substantial authority isn’t only a taxpayer concern. A tax return preparer faces a separate penalty under Section 6694 for an unreasonable position. For undisclosed positions, the preparer must meet the same substantial authority standard; the penalty is the greater of $1,000 or 50 percent of the income the preparer earned from that return.⁸ For disclosed positions the threshold drops to reasonable basis, and for tax shelters and reportable transactions it rises to more likely than not.

Because of that exposure, a competent preparer should raise the substantial authority question with you before signing an aggressive return, and should tell you about penalties that are reasonably likely to apply and any chance to avoid them through disclosure.⁹ If the phrase never comes up when the position is aggressive, ask.

1
eCFR. 26 CFR 1.6662-4 – Substantial Understatement of Income Tax
2
Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments
3
GovInfo. 26 USC 6662A – Imposition of Accuracy-Related Penalty on Understatements With Respect to Reportable Transactions
4
Internal Revenue Service. Instructions for Form 8275
5
Internal Revenue Service. Instructions for Form 8275-R
6
Office of the Law Revision Counsel. 26 USC 6664 – Definitions and Special Rules
7
Internal Revenue Service. Reasonable Cause and Good Faith
8
Office of the Law Revision Counsel. 26 USC 6694 – Understatement of Taxpayers Liability by Tax Return Preparer
9
eCFR. 31 CFR 10.34 – Standards With Respect to Tax Returns and Documents, Affidavits and Other Papers