An IRS Notice of Deficiency is a formal letter telling you the IRS believes you owe more tax than you reported for a specific year, and it starts a 90-day countdown to challenge that amount in U.S. Tax Court before the IRS can assess the balance and begin collecting. The deadline is set by federal statute and cannot be extended for any reason.1Office of the Law Revision Counsel. 26 USC 6213 – Restrictions Applicable to Deficiencies; Petition to Tax Court What you do in those 90 days decides whether you keep the right to fight the bill without paying it first.
What the Notice Is, and What It Isn’t
A Notice of Deficiency is the IRS’s official legal determination that your tax liability exceeds what you reported. The IRS must send it by certified or registered mail before it can formally assess additional income tax against you.2GovInfo. 26 USC 6212 – Notice of Deficiency Until the response window closes or a Tax Court petition is resolved, the IRS is legally prohibited from assessing the deficiency or starting collection.1Office of the Law Revision Counsel. 26 USC 6213 – Restrictions Applicable to Deficiencies; Petition to Tax Court
So the notice is a proposal. You don’t owe anything yet, and no payment is due while the 90 days are running. It usually arrives as Form CP3219A (from the Automated Underreporter program) or Letter 3219 (from an audit). Both carry the same legal weight and start the same clock.3Taxpayer Advocate Service. 90 Day Notice of Deficiency
The notice sets out the tax year, the additional tax the IRS proposes, and any penalties and interest. It also explains the specific adjustments, such as income from a particular 1099, a disallowed deduction, or a recalculated credit, and it must list contact information for your local Taxpayer Advocate office.2GovInfo. 26 USC 6212 – Notice of Deficiency An enclosed Form 5564 (Notice of Deficiency–Waiver) is for signing only if you agree with the proposed amount.4Internal Revenue Service. Understanding Your CP3219A Notice
The 90-Day Deadline Is Absolute
You have 90 days from the mailing date printed on the notice to file a petition with the U.S. Tax Court. If you are outside the United States, the window is 150 days. If the 90th day falls on a Saturday, Sunday, or legal holiday in the District of Columbia, you get the next business day.1Office of the Law Revision Counsel. 26 USC 6213 – Restrictions Applicable to Deficiencies; Petition to Tax Court
The clock starts on the mailing date, not the day you open the envelope. The IRS Internal Revenue Manual states there is no provision in law for suspending or restarting that period.5Internal Revenue Service. Internal Revenue Manual 4.8.9 – Statutory Notices of Deficiency Illness, travel, a tax preparer who sat on it — none of that extends the deadline.
The Last Known Address Trap
The notice is legally valid as long as the IRS mails it to your “last known address,” even if you never actually receive it. Under Treasury regulations, your last known address is the address on your most recently filed and properly processed return, unless you’ve given the IRS clear notification of a new one.6Internal Revenue Service. Definition of Last Known Address (Rev. Proc. 2010-16) The IRS also picks up address changes from the USPS National Change of Address database, but that system isn’t foolproof.
If you’ve moved since filing your last return, update your address with the IRS using Form 8822. A notice sent to an outdated address still starts the 90-day clock, and missing the deadline because the letter went to an old apartment means losing your right to Tax Court.
Why You Got One
The most common trigger is an income mismatch. Employers, banks, and brokerages report what they paid you on W-2s and 1099s, and the IRS cross-checks those numbers against your return. If income appears on a 1099 but not on your filing, the Automated Underreporter system eventually generates a CP3219A proposing additional tax on the gap.
Audit findings are the second path. If an examiner proposes changes you disagree with or never respond to, the examiner issues a Notice of Deficiency reflecting those adjustments.
A third scenario is an unfiled return. When you don’t file a required return, the IRS can prepare a “substitute for return” based on income information from third parties.7Office of the Law Revision Counsel. 26 USC 6020 – Returns Prepared for or Executed by Secretary Substitutes typically allow only the standard deduction and no credits, so the resulting bill is almost always higher than what you’d owe on a properly prepared return.
Math errors on your original return can also lead here, though they usually produce a separate “math error notice” first with a 60-day window to request reversal. If that period passes without resolution, the IRS may follow up with a formal Notice of Deficiency.8Taxpayer Advocate Service. Math Error Notices: What You Need to Know and What the IRS Needs to Do to Improve Notices
Before you decide anything else, read the explanation of changes line by line. The IRS sometimes gets it wrong, especially with income matching. A 1099 might report the gross amount of a stock sale while your return correctly reported only the net gain, and the notice won’t reflect that distinction unless you point it out. Pulling your IRS account transcript for the year in question will show the income documents the IRS has on file and can confirm whether a mismatch is real or a payer’s reporting error. You can request transcripts at irs.gov or by calling the IRS.
Your Three Options
Agree and Pay
If the IRS is right, sign and return Form 5564 to accept the additional tax. You can include payment with the form or wait for a bill.9Taxpayer Advocate Service. Notice CP3219A – Automated Under Reporter (AUR) Notice of Deficiency Paying sooner limits the interest, which runs at 7% per year, compounded daily, as of early 2026.10Internal Revenue Service. Interest Rates Remain the Same for the First Quarter of 2026
Petition the U.S. Tax Court
If you disagree, you can challenge the deficiency in U.S. Tax Court without paying the disputed amount first. This is the only court where you can fight a proposed tax bill before paying. The petition must be filed within 90 days of the notice’s mailing date (150 days if you’re outside the country).1Office of the Law Revision Counsel. 26 USC 6213 – Restrictions Applicable to Deficiencies; Petition to Tax Court
The filing fee is $60, payable to “Clerk, United States Tax Court.” The court can waive the fee if you show financial hardship through a sworn application. You can file electronically through the court’s DAWSON system or mail a paper petition to the Tax Court in Washington, D.C.11United States Tax Court. Guidance for Petitioners: Starting A Case Your petition must include a copy of the Notice of Deficiency (with Social Security numbers redacted), a Statement of Taxpayer Identification Number (Form 4), a Request for Place of Trial (Form 5), and the fee or waiver application. Don’t send tax returns, receipts, or other evidence with the petition itself.12United States Tax Court. Petition (Simplified Form 2)
If the disputed amount for a given tax year is $50,000 or less, you can elect “small tax case” (S case) procedures. The process is simpler and more informal, and it’s designed for people without attorneys. The trade-off is that an S case decision is final and cannot be appealed by either side.13Office of the Law Revision Counsel. 26 USC 7463 – Disputes Involving $50,000 or Less
Pay First, Then Sue for a Refund
A third path is to pay the deficiency in full and then file a claim for refund. This makes sense if you’ve missed the Tax Court deadline or prefer to litigate in federal district court or the Court of Federal Claims instead. The refund claim must be filed within three years of filing the original return or two years of paying the tax, whichever is later.14Internal Revenue Service. Refund Litigation If the IRS denies the claim or doesn’t act on it within six months, you can sue. This route requires paying the full amount upfront, which is why most people prefer Tax Court when the deadline still allows it.
What Happens If You Ignore the Notice
Once the 90-day window closes without a petition, the IRS assesses the full proposed amount as if you agreed. Three things happen at once: you lose access to Tax Court, the balance becomes legally enforceable, and collection begins. You’ll receive balance-due notices, and eventually a notice of intent to levy and a notice of federal tax lien.15Internal Revenue Service. Topic No. 201 The Collection Process Liens attach to everything you own, including property you acquire after the lien is filed. Levies can reach bank accounts and wages.
The financial damage compounds. The failure-to-pay penalty is 0.5% of the unpaid balance per month, capped at 25%, and rises to 1% per month after the IRS issues a final notice of intent to levy.16Internal Revenue Service. Failure to Pay Penalty Interest at 7% per year, compounded daily, runs on top of that.10Internal Revenue Service. Interest Rates Remain the Same for the First Quarter of 2026 If the deficiency stems from negligence or a substantial understatement of income, the IRS can also add an accuracy-related penalty of 20% of the underpayment.17Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments
There is one later safeguard, but it’s narrow. Before the IRS can seize property, it must send a notice of intent to levy at least 30 days in advance, and you can request a Collection Due Process hearing with the IRS Independent Office of Appeals.18Office of the Law Revision Counsel. 26 USC 6330 – Notice and Opportunity for Hearing Before Levy At a CDP hearing you can propose alternatives like an installment agreement or an offer in compromise, but you can challenge the underlying tax amount only if you had no prior opportunity to dispute it. Receiving (or being deemed to have received) a valid Notice of Deficiency generally counts as that prior opportunity, so CDP is not a substitute for responding on time.
Joint Returns and Innocent Spouse Relief
When the notice is based on a joint return, both spouses are on the hook for the entire amount. If the underreported income or erroneous deduction belongs entirely to your spouse or former spouse, you can request innocent spouse relief by filing Form 8857.19Internal Revenue Service. About Form 8857, Request for Innocent Spouse Relief The IRS evaluates whether you knew or should have known about the understatement and whether holding you liable would be unfair. File Form 8857 as soon as you become aware of the liability; the general deadline is no later than two years after the IRS first begins collection activity against you.20Internal Revenue Service. Publication 971 (12/2021), Innocent Spouse Relief
When spouses have established separate residences and the IRS has been notified, the agency must send a separate copy of the Notice of Deficiency to each spouse at their respective addresses.2GovInfo. 26 USC 6212 – Notice of Deficiency
Getting Help
For deficiencies above a few thousand dollars, professional representation often pays for itself. Enrolled agents, CPAs, and tax attorneys can all represent you before the IRS and in Tax Court. If you can’t afford professional help, Low Income Taxpayer Clinics funded by the IRS provide free or low-cost assistance to qualifying taxpayers. Whatever you decide, calendar the 90-day deadline the day the notice arrives and work backward from there.