Notice of Reassessment: How to Respond, Deadlines, and Penalties

A notice of reassessment from the IRS isn’t a single document. The agency uses several notices to tell you your return has been reviewed and your tax has changed, and each one carries a different deadline and a different set of rights. The three you’re most likely to see are the CP2000, which proposes changes based on income the IRS thinks you underreported; a math error notice, which corrects arithmetic or clerical mistakes; and a Statutory Notice of Deficiency, the formal 90-day letter that starts your clock to petition Tax Court. Before you do anything else, find the CP or LTR number in the upper right corner of the letter and the date printed on the first page. Those two pieces of information decide everything that follows.

Which Notice Did You Get

CP2000 (Proposed Changes)

The CP2000 is the most common. It comes out of the IRS’s Automated Underreporter program, which matches income reported by employers, banks, and brokerages against what you put on your return. It proposes adjustments; it is not a bill. You have 30 days from the notice date to respond, or 60 days if you live outside the United States.¹ You can agree, partially agree, or disagree. Responses go back through the IRS’s secure upload portal, by fax, or by mail to the address on the notice.²

Math Error Notice

When the IRS finds an arithmetic or clerical error, it can assess the extra tax immediately without a formal deficiency notice. You still get a right to push back: within 60 days of the notice date, you can request abatement of the assessment. If you request abatement inside that window, the IRS must reverse the assessment and can only pursue the tax through the regular deficiency procedures, which include your right to petition Tax Court.³ The IRS cannot levy during those 60 days.

Statutory Notice of Deficiency (90-Day Letter)

This is the one to take most seriously. It arrives by certified or registered mail and formally determines you owe additional tax. From the mailing date you have exactly 90 days to file a petition with the U.S. Tax Court, or 150 days if the notice is addressed outside the United States.³ The deadline is set by statute, and the IRS cannot extend it.⁴ Miss it, and the IRS assesses the deficiency and moves to collection.

How to Read the Notice

Every reassessment notice lays out a side-by-side comparison of what you reported and what the IRS says the numbers should be. Focus on the line-by-line changes: adjusted gross income, specific deductions, credits, or income items the IRS says you left off. That’s where your response needs to land.

The explanation section tells you why each change was made. On a CP2000, this usually names the specific payer whose reported figure doesn’t match your return, which is your fastest clue to what documentation you’ll need. Check the bottom line, too. Occasionally an adjustment cuts the other way and produces a larger refund.

Two things on the first page govern the rest. The notice date starts your response clock. The notice number (CP or LTR followed by digits) tells you which procedure applies. Put the case or document reference number on everything you send back so it reaches the right file.

How to Respond

Your options depend on how much of the IRS’s version you accept.

If the IRS is right, sign the response form, pay the balance or arrange a payment plan, and close it out. You don’t need to file an amended return if you agree with a CP2000 and have no other changes to report.²

If you agree with some items but not others, mark which you accept and which you’re disputing. Pay the undisputed portion to stop interest running on it, and send documentation only for the items you’re contesting.

If you disagree entirely, write a clear explanation of why each proposed change is wrong and attach evidence. Match your documents to the specific line numbers in the IRS’s explanation. Assertions without paperwork rarely move the case.

Requesting an Appeals Review

If the examiner sticks with the proposed changes after reading your response, you can ask the IRS Independent Office of Appeals to look at the case. For proposed adjustments of $25,000 or less per tax year, the vehicle is Form 12203 (Request for Appeals Review).⁵ You’ll list the items you disagree with and your reasons. If someone is representing you, attach Form 2848 (Power of Attorney). Appeals officers work independently of the examining function and can settle cases without going to court. Expect an initial response within about 30 days, though the full review usually runs longer.

Documentation That Actually Works

The IRS decides these disputes on records, not narrative. What you send needs to directly contradict the proposed change, and it should be contemporaneous, meaning created around the time of the transaction rather than reconstructed after the notice arrived.

Unreported income: the actual W-2 or 1099 showing the correct amount, bank records reconciling to what you reported, or a corrected information return from the payer if the original was wrong.
Disallowed deductions: receipts, invoices, cancelled checks, or credit card statements showing the expense was paid during the tax year. For business expenses, a contemporaneous mileage log or expense diary carries far more weight than a spreadsheet built after the fact.
Credit disputes: proof of eligibility, such as school enrollment statements for education credits or residency records for a dependent claimed under the Child Tax Credit.

Order your evidence to track the line numbers in the notice. Send it as one organized package. Missing documentation is the single most common reason these disputes fail.

How Long the IRS Has to Reassess

The assessment window is a defense worth checking, especially on older tax years.

Three years is the general rule, running from the date you filed the return.⁶
Six years applies if you omitted more than 25% of the gross income shown on your return.⁶
There’s no time limit if the return was fraudulent or was never filed.⁶

The clock runs from the actual filing date, so a return filed late has a later expiration. If the notice is proposing changes to a return that’s outside the applicable window, raise the statute of limitations directly in your response.

What It Will Cost

A reassessment that increases your tax almost always brings interest, and it may bring penalties.

Interest

Interest runs on unpaid tax from the original due date of the return, not from the notice date. The rate is reset quarterly. For 2026 it’s 7% in the first quarter and 6% in the second quarter for individual underpayments.⁷ It compounds daily, and unlike penalties, it generally cannot be abated.

Failure-to-Pay Penalty

If the reassessment leaves an unpaid balance, the failure-to-pay penalty runs at 0.5% of the unpaid tax per month or partial month, up to 25%. That drops to 0.25% per month once you’re in an approved installment agreement. It rises to 1% per month if you get a notice of intent to levy and don’t pay within 10 days.⁸

Accuracy-Related Penalty

If the underpayment came from negligence or a substantial understatement, the IRS can add an accuracy-related penalty of 20% of the underpayment. For individuals, a substantial understatement means the correct tax exceeds what you reported by the greater of 10% of the correct tax or $5,000.⁹ On a large adjustment this is where the bill really grows.

First-Time Abatement

If your compliance record is clean, you may qualify for First-Time Abatement, which removes one qualifying penalty (failure to file, failure to pay, or failure to deposit) for a single tax period. You must have filed all required returns of the same type for the prior three years and had no penalties during that period, or any prior penalty must have been removed for a reason other than First-Time Abatement.¹⁰ It doesn’t touch the underlying tax or the interest, but it can meaningfully reduce the bill. Request it by phone or in writing as part of your notice response.

If You Can’t Pay in Full

Agreeing with the reassessment doesn’t mean you have to write a check for the whole balance today. A short-term plan gives individuals up to 180 days to pay, with no setup fee.¹¹

A long-term installment agreement stretches payments further. Setup fees vary by application method and payment type:

Direct debit: $22 online, $107 by phone or mail.¹¹
Other payment methods: $69 online, $178 by phone or mail.¹¹
Low-income taxpayers: the fee is waived entirely for direct debit agreements. For other methods, the fee is $43 and may be reimbursed when the plan is complete.¹¹

While an installment agreement request is pending, the IRS is generally prohibited from levying, and the collection period is suspended. That protection continues for 30 days after a rejection, giving you time to appeal.¹¹ Interest and the reduced failure-to-pay penalty keep accruing during the plan, so faster payoff means less total cost.

If You Ignore It

Ignoring the notice is the most expensive outcome. If you don’t respond to a CP2000 in time, the IRS sends a Statutory Notice of Deficiency.¹ If you also let the 90-day petition window lapse, the deficiency is assessed and collection begins.¹² From there the IRS can file a federal tax lien, levy bank accounts, or garnish wages.

The stack builds fast: the full proposed deficiency, potentially a 20% accuracy-related penalty, a failure-to-pay penalty of up to 25% over time, and daily-compounding interest running from the original return due date. Even if you can’t pay, responding to the notice keeps your options open, including installment agreements and first-time penalty relief that are much harder to access once the case has moved into forced collection.

1
Internal Revenue Service. Topic No. 652, Notice of Underreported Income – CP2000
2
Internal Revenue Service. Understanding Your CP2000 Series Notice
3
Office of the Law Revision Counsel. 26 USC 6213 – Restrictions Applicable to Deficiencies; Petition to Tax Court
4
Taxpayer Advocate Service. 90 Day Notice of Deficiency
5
Internal Revenue Service. Request for Appeals Review
6
Office of the Law Revision Counsel. 26 USC 6501 – Limitations on Assessment and Collection
7
Internal Revenue Service. Quarterly Interest Rates
8
Internal Revenue Service. Failure to Pay Penalty
9
Internal Revenue Service. Accuracy-Related Penalty
10
Internal Revenue Service. Administrative Penalty Relief
11
Internal Revenue Service. Payment Plans; Installment Agreements
12
Internal Revenue Service. IRS IRM 4.8.9 Statutory Notices of Deficiency