IRS Statute of Limitations on Audits: The 3-Year, 6-Year, and Forever Rules

The IRS does not have unlimited time to audit your return. The Internal Revenue Code sets out specific statutes of limitations that, once expired, bar the IRS from assessing additional tax. Knowing exactly when your statute runs — and what can extend it — is one of the most important pieces of an audit defense.

The general rule: three years

Under IRC § 6501(a), the IRS generally has three years from the date the return was filed to assess additional tax. If you filed early, the clock starts on the original due date (April 15 for most individual returns). If you filed late, the clock starts on the actual filing date.

This is the statute most taxpayers are familiar with, and it applies to most audits. By the time the IRS issues a 90-day letter, the statute must still be open — or the assessment is invalid.

The six-year rule: substantial omission

If you omit more than 25% of your gross income from the return, the statute extends to six years under § 6501(e). “Omission” has a specific technical meaning — it isn’t simply any error. The income must be completely absent from the return, not just under-reported in amount. Disclosed positions, even if wrong, typically don’t trigger the six-year rule.

The six-year rule also applies to omissions of more than $5,000 in foreign financial assets.

No statute: fraud and unfiled returns

Two situations leave the IRS with unlimited time:

• Fraud. If the IRS proves the return was filed with intent to evade tax, there is no statute of limitations. The IRS bears the burden of proving fraud by clear and convincing evidence — see IRS Audit Penalties for more.
• No return filed. If you never filed a return for a given year, the statute never starts. The IRS can come after that year decades later.

Form 872: extending the statute by agreement

Late in an audit, the IRS frequently asks taxpayers to sign Form 872 (a consent to extend the assessment statute). The IRS uses the extra time to finish its work; in exchange, the taxpayer gets more time to respond, gather records, and consider Appeals. Signing is voluntary. If you refuse, the IRS will typically issue a hurried Notice of Deficiency to protect the statute.

Whether to sign is a judgment call. A scoped, limited-issue 872 (with specific end date and limited to specific issues) is usually fine. An open-ended consent is rarely a good idea.

Collection statute: 10 years

Separate from the assessment statute, the IRS has 10 years from the date of assessment to collect the tax under § 6502. This is called the Collection Statute Expiration Date (CSED). Once it expires, the tax debt is legally extinguished. Certain events — like bankruptcy, an Offer in Compromise, or a Collection Due Process hearing — toll (pause) the CSED.

Refund claim statute

To claim a refund, you generally have the later of three years from filing or two years from payment under § 6511. The amount of refund is also capped by the look-back period. This often matters when an audit produces both adjustments going up and down — if the refund statute is closed for a downward adjustment, you may lose the benefit.

Why this matters in an audit

When the IRS opens an audit late in the cycle — say 30 months after filing — the statute is a major strategic consideration. The IRS will be working against the clock, which often translates into pressure to sign Form 872 or settle quickly. Knowing the rules lets you push back instead of accepting the IRS’s preferred outcome.

For more on the overall flow of an audit, see What Happens During an IRS Audit and IRS Audit Timeline.