Reasonable Basis Test: Confidence Levels, Disclosure, and Penalties

The reasonable basis test is the minimum level of legal support a position on your tax return needs before the IRS’s accuracy-related penalty can be kept off your bill. It sits at roughly a 20% chance that the position would be upheld on the merits: more than a creative-sounding argument, less than the higher “substantial authority” standard. On its own it does little; combined with proper disclosure on your return, it defeats the 20% penalty the IRS otherwise adds when it disagrees with how you reported an item.

What the Standard Actually Requires

A position has a reasonable basis when it rests on one or more recognized legal authorities and the link between those authorities and your tax treatment is logical rather than merely conceivable. The IRS describes the bar as “significantly higher than not frivolous.” A colorable argument invented to gain leverage in an audit won’t clear it.

Treasury Decision 8790, which finalized the current definition, confirmed that a position satisfies the standard when it is “reasonably based on one or more” of the authorities the regulations list.¹ The list of acceptable authorities is the same one used for the higher substantial authority standard. What changes between the two tests is how much weight those authorities need to carry. For reasonable basis, even a single relevant authority can be enough.

Courts have split on whether the test looks only at what the authorities say (objective) or also at whether you actually consulted them before filing (subjective). In Wells Fargo & Co. v. United States, the Eighth Circuit held that the defense “requires evidence that a taxpayer actually relied on the relevant legal authorities that form the reasonable basis for its position.”² Whether other circuits will follow is unresolved. The practical response is to keep a record of the authorities you reviewed before filing.

Where Reasonable Basis Sits in the Confidence Hierarchy

Tax law uses a sliding scale of confidence levels. Knowing where reasonable basis falls tells you when it’s enough and when it isn’t.

Not frivolous: the floor. The position isn’t patently absurd. Protects nothing on its own.
Reasonable basis: about 20% confidence. Avoids negligence penalties, and avoids the substantial understatement penalty when paired with disclosure.
Substantial authority: about 40% confidence. Avoids the substantial understatement penalty even without disclosure.
More likely than not: greater than 50% confidence. Required for tax shelter and reportable transaction positions.

Reasonable basis is entry-level protection. Without disclosure, it isn’t enough by itself to block a substantial understatement penalty; you’d need substantial authority for that.³

Which Authorities Count

Not every source you might read counts as a “legal authority” for this test. The Treasury Regulations set out an exhaustive list, and anything outside it carries no weight during an audit.⁴ The recognized authorities are:

The Internal Revenue Code and proposed, temporary, and final Treasury Regulations.
Revenue rulings, revenue procedures, notices, announcements, and press releases published in the Internal Revenue Bulletin.
Decisions from the U.S. Tax Court, the Court of Federal Claims, district courts, and appellate courts.
Legislative history, including committee reports, conference explanations, floor statements by a bill’s managers before enactment, and the Joint Committee on Taxation’s General Explanations (the “Blue Book”).
Private letter rulings and technical advice memoranda issued after October 31, 1976, plus actions on decisions and general counsel memoranda issued after March 12, 1981.
Tax treaties, regulations interpreting them, and official Treasury Department explanations.

Notice what’s missing. IRS publications, FAQ pages on the IRS website, tax software help articles, professional newsletters, blog posts, and commercial tax guides do not count. They may help you understand the law, but if the IRS challenges your position, pointing to a Publication or a TurboTax help page carries no weight. The reasoning has to trace back to one of the sources on the list.

The Penalties It Protects Against

The accuracy-related penalty under Section 6662 adds 20% to the portion of your underpayment that triggers it.³ Two categories of error are where reasonable basis does most of its work.

Negligence

Negligence is the failure to make a reasonable attempt to comply with the tax code. If the IRS charges negligence, having a reasonable basis for the position and adequately disclosing it defeats the claim. Grounding the position in recognized authority and telling the IRS about it isn’t carelessness, even if the IRS ultimately disagrees.

Substantial Understatements

A substantial understatement exists when your understatement exceeds the greater of 10% of the tax that should have been on your return or $5,000. Corporations use a different threshold: the lesser of 10% of the correct tax (or $10,000 if greater) and $10 million. If you claimed the Section 199A qualified business income deduction, the percentage drops from 10% to 5%.³

The understatement amount is reduced, potentially below the threshold, if the position either had substantial authority on its own or had a reasonable basis and you adequately disclosed the relevant facts. That second route is the accessible one for most taxpayers, but both pieces are required: legal support and disclosure.

Penalty protection does not erase the tax itself. You still owe the underlying tax and interest. What reasonable basis plus disclosure keeps off is the additional 20%.

How to Disclose: Form 8275 and Form 8275-R

The disclosure piece runs through Form 8275, filed with your original return. If the position contradicts a specific Treasury Regulation, use Form 8275-R instead.⁵

Checking a box isn’t enough. The form asks you to identify the specific rule or statutory provision, describe the item and its dollar amount, specify where it appears on your return, and describe the relevant facts. The instructions state that disclosure is adequate only if it gives the IRS enough information to identify the item, its amount, and the nature of the potential controversy.⁵ Writing “deduction for business expenses” without pinpointing the treatment you’re defending doesn’t put the IRS on notice of anything.

If You Didn’t Disclose on the Original Return

You can still attach Form 8275 to a qualified amended return, but the window is short. The amended return must be filed before the earliest of several triggering events, the most common being the date the IRS first contacts you about examining the return.⁶ Other triggers include IRS contact with a pass-through entity whose items flow to your return and the service of a John Doe summons related to your transactions. Once an audit letter arrives, that door is closed.

Where Reasonable Basis Does Not Apply

The disclosure-plus-reasonable-basis defense has a firm limit: it does not work for tax shelters. Section 6662(d)(2)(C) defines a tax shelter to include any partnership, entity, investment plan, or arrangement whose significant purpose is avoiding or evading federal income tax. If a transaction is classified that way, the understatement reduction for adequate disclosure and reasonable basis disappears.³

The same limit applies to reportable transactions subject to Section 6662A. For both, the minimum required standard jumps to “more likely than not,” meaning greater than 50% confidence in the position. Separate penalties under Section 6707A apply if a reportable transaction isn’t disclosed at all.⁷ A common mistake is assuming reasonable basis covers an aggressive strategy that turns out to fall inside the tax shelter definition.

Reasonable Basis Compared With Reasonable Cause

The two defenses sound similar and get confused often. Reasonable basis looks at the legal authorities behind the position. Reasonable cause, under Section 6664(c), looks at your conduct: whether you exercised ordinary business care and acted in good faith.⁸

Reasonable cause can eliminate the accuracy-related penalty entirely, even without disclosure, if you show good faith and ordinary care. It can cover ground reasonable basis doesn’t, such as reliance on professional advice that turned out to be wrong. Reasonable cause does not apply to penalties for transactions lacking economic substance under Section 6662(b)(6), and it has special conditions for overvalued charitable deductions, including a requirement for a qualified appraisal and a good-faith investigation of value.⁸

The two are not mutually exclusive. If reasonable basis plus disclosure doesn’t fully cover you, reasonable cause may still work as a backup.

Why Your Preparer Cares Too

Tax preparers face their own penalties tied to this standard. Under Section 6694(a), a preparer who knew or reasonably should have known a return contained an unreasonable position faces a penalty equal to the greater of $1,000 or 50% of the income earned from preparing the return.⁹

The applicable standard depends on disclosure. For undisclosed positions, the preparer needs substantial authority. For disclosed positions on Form 8275 or 8275-R, reasonable basis is enough. For tax shelters and reportable transactions, the bar is “more likely than not” whether disclosed or not. Treasury Circular 230 reinforces the same expectations for attorneys, CPAs, and enrolled agents, prohibiting a practitioner from willfully, recklessly, or through gross incompetence signing a return that lacks a reasonable basis.¹⁰

The alignment is useful. If a CPA or tax attorney refuses to sign off on a position because they cannot identify a reasonable basis for it, that refusal is a strong signal about how the position would fare in an audit.

1
Internal Revenue Service. TD 8790 – Definition of Reasonable Basis
2
U.S. Court of Appeals for the Eighth Circuit. Wells Fargo & Co. v. United States
3
Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments
4
eCFR. 26 CFR 1.6662-4 – Substantial Understatement of Income Tax
5
Internal Revenue Service. Instructions for Form 8275 (10/2024)
6
Internal Revenue Service. Qualified Amended Returns – Notice 2004-38
7
Office of the Law Revision Counsel. 26 USC 6707A – Penalty for Failure To Include Reportable Transaction Information With Return
8
Office of the Law Revision Counsel. 26 USC 6664 – Definitions and Special Rules
9
Office of the Law Revision Counsel. 26 USC 6694 – Understatement of Taxpayer’s Liability by Tax Return Preparer
10
eCFR. 31 CFR 10.34 – Standards With Respect to Tax Returns and Documents, Affidavits, and Other Papers