IRS Statute of Limitations: The 3-Year, 6-Year, 10-Year, and Unlimited Rules

The IRS does not have unlimited time to audit your return or collect your tax debt. Four statutory clocks govern IRS enforcement, and each has different rules, different triggers, and different exceptions. The 3-year assessment period is the general rule: the IRS must assess additional tax within three years of the return being filed. The 6-year rule applies when more than 25% of gross income is omitted. The 10-year collection period (CSED) governs how long the IRS can collect an assessed debt. And the unlimited rule applies to fraud and unfiled returns. Understanding which clock applies, when it started, and what can pause or restart it is the difference between owing nothing and owing everything.

What is the 3-year assessment period?

Under IRC 6501(a), the IRS must assess additional tax within three years after the return was filed. “Filed” means the later of: the actual filing date, or the due date of the return (including extensions). If you file your 2023 return on March 1, 2024, the clock starts April 15, 2024 (the due date), and the IRS has until April 15, 2027 to assess additional tax. If you file on extension and submit the return October 10, 2024, the clock starts October 10, 2024, and runs to October 10, 2027.

After the three-year period expires, the IRS cannot assess additional tax, penalties, or interest for that year, even if it discovers a clear error. The assessment statute of limitations is a hard bar.

Filing early does not start the clock early. A return filed on January 15 is treated as filed on April 15 (the due date) for statute purposes. Filing on extension starts the clock on the actual filing date.

An amended return (Form 1040-X) does not restart the three-year clock for the original return. The IRS can assess tax on items shown on the amended return within three years of the amended return’s filing date, but only to the extent the amended return changed the original.

What is the 6-year rule for substantial omissions?

Under IRC 6501(e)(1)(A), the assessment period extends to six years when the taxpayer omits more than 25% of the gross income stated on the return. “Gross income” means total income before deductions, not adjusted gross income or taxable income.

For example: if your return reports $80,000 in gross income and you omitted $25,000 in income from a side business, the omission ($25,000) exceeds 25% of reported gross income ($20,000). The six-year rule applies, giving the IRS until six years after filing to assess the additional tax.

The 25% threshold applies to gross income reported on the return, not to the correct gross income. The IRS must show that the omitted amount exceeds 25% of what was reported.

For cross-border filers, the 6-year rule frequently applies because foreign income omissions are common (Canadian investment income not reported on the US return, foreign pension income, foreign rental income). If the omission pushes past the 25% threshold, the IRS has three additional years to assess.

Basis overstatements. The Supreme Court’s 2012 decision in United States v. Home Concrete & Supply held that an overstatement of basis (which reduces reported gain without omitting the gross proceeds) is not an “omission” for purposes of the 6-year rule. Congress legislatively reversed this for returns filed after July 31, 2015, by amending IRC 6501(e)(1)(B)(ii) to treat basis overstatements as omissions.

When is the statute unlimited?

The IRS has no time limit for assessment in two situations:

Fraud. Under IRC 6501(c)(1), if the return was filed with the intent to evade tax, the IRS can assess additional tax at any time. There is no three-year, six-year, or any other limit. The IRS must prove fraud (an intentional wrongdoing designed to evade tax), not just negligence or error. See IRS criminal vs civil investigation for the distinction.

Unfiled returns. Under IRC 6501(c)(3), if no return was filed, the IRS can assess tax at any time. Filing a delinquent return starts the three-year clock. Until the return is filed, the clock never starts. This is why the IRS can assess tax for a year in which a return was due 20 years ago and never filed.

For US citizens living in Canada who have never filed US returns, this means the IRS can theoretically assess tax for every unfiled year going back to the beginning of the obligation. In practice, the IRS’s Streamlined Filing Compliance Procedures require only three years of returns and six years of FBARs, and the delinquent return procedures also focus on recent years. But the legal authority to assess older years exists.

What is the 10-year collection period (CSED)?

Once the IRS assesses a tax (issues a notice establishing the liability), the Collection Statute Expiration Date (CSED) begins. Under IRC 6502, the IRS has 10 years from the date of assessment to collect the tax. After the CSED, the debt is legally extinguished, and the IRS cannot levy, lien, garnish, or otherwise collect.

The CSED is the single most important date for taxpayers in IRS collections. A $100,000 debt with 2 years left on the CSED is a fundamentally different problem than a $100,000 debt with 9 years left. The collection strategy (installment agreement, OIC, Currently Not Collectible, or just waiting) depends heavily on where the CSED stands.

You can check your CSED by requesting transcripts from the IRS (the CSED is shown on the account transcript for each assessment). The IRS will not volunteer this information, and revenue officers are not required to tell you how much time is left on the collection statute.

What tolls (pauses) the assessment clock?

Several events pause the three-year or six-year assessment period:

Consent to extend (Form 872). The IRS can ask you to sign Form 872 (Consent to Extend the Time to Assess Tax), extending the assessment period for a specific tax year. This is common during audits: the IRS needs more time to complete the examination and asks for an extension rather than rushing to assess. You can refuse, but the IRS may then assess based on incomplete information (usually unfavorable to you). You can also negotiate a restricted consent (Form 872-A) that limits the extension to specific issues.

Tax Court petition. Filing a petition in Tax Court suspends the assessment period during the proceeding and for 60 days after the decision becomes final.

Request for prompt assessment. An executor can request a prompt assessment under IRC 6501(d), which shortens the period to 18 months from the request.

What tolls (pauses) the collection clock?

Multiple events pause the 10-year CSED:

Offer in Compromise (OIC). Filing an OIC suspends the CSED while the offer is pending and for 30 days after rejection (or while an appeal of the rejection is pending). A rejected OIC that took 18 months to process extends the CSED by approximately 19 months.

Installment agreement. An active installment agreement tolls the CSED during the agreement and for 30 days after termination. This is the biggest trap: a 6-year installment agreement that defaults can extend the CSED by 6+ years, giving the IRS more time to collect than if no agreement had been entered.

Collection Due Process (CDP) hearing. Requesting a CDP hearing suspends the CSED while the hearing is pending and during any Tax Court proceeding that follows.

Bankruptcy. Filing for bankruptcy suspends the CSED during the automatic stay and for 6 months after the stay is lifted.

Absence from the US. Under IRC 6503(c), if the taxpayer is continuously absent from the United States for six months or more, the CSED is suspended during the absence. For Canadian residents with IRS debt, this means the 10-year clock pauses for the entire time they are outside the US. A taxpayer who moves to Canada with 7 years left on the CSED and stays for 10 years comes back to 7 years remaining.

Military service. Active military service in a combat zone suspends the CSED.

How does the absence-from-US rule affect cross-border filers?

IRC 6503(c) is the provision that catches cross-border filers. The IRS’s position: if you are outside the US for a continuous period of six months or more, the collection statute is suspended for the entire period of absence. For a Canadian resident with an IRS debt of $50,000 and a CSED that would otherwise expire in 2030, moving to Canada in 2025 pauses the clock. The debt does not expire while the taxpayer lives in Canada, regardless of how many years pass.

The IRS can still collect from US-based assets (bank accounts, real estate, retirement accounts) while the taxpayer is abroad. For collection from Canadian assets, the IRS would need to invoke the treaty collection provisions (Article XXVIA), which require the debt to be “finally determined” and carry procedural requirements.

The practical implication: a Canadian resident cannot simply wait out the 10-year CSED by living in Canada. The CSED pauses for the duration of the absence. The collection strategies that work for domestic taxpayers (waiting for the CSED to expire while making minimum payments) do not work for taxpayers who live abroad.

What should I do next?

Determine which statutes apply to your situation. For assessment: if you filed the return more than three years ago and there is no substantial omission or fraud, the IRS generally cannot assess additional tax. For collection: check the CSED on your IRS account transcript and identify any tolling events that may have extended it. If you are a Canadian resident with IRS debt, the absence-from-US rule likely pauses the CSED. Factor this into your collection strategy when evaluating an installment agreement, OIC, or CNC.

This guide is general information, not tax advice for your specific situation. Which points apply, and how, depends on your facts.