What the notice means
The IRS CP521 page describes it in one line: “the IRS is reminding you that you have an installment payment due. Send your payment immediately.” The notice shows the amount due, the due date and the remaining balance, with a tear-off stub and a pre-addressed envelope. The IRS FAQ on the page says: “If you don’t send us a payment, the interest and penalties you’ll have to pay will increase and you could default on your installment agreement.”
The notice has Spanish (CP521 SP) and business (CP621) versions. It is informational; nothing is proposed, nothing is assessed and there is no appeal. Its value to a practitioner is as a monthly check that the agreement is still in status 60 and that payments are posting to the right periods.
Why the client got it
IRM 5.19.1.6.4 defines the arrangement: installment agreements “are arrangements by which the IRS allows taxpayers to pay liabilities over time,” and “During the course of IAs, penalties and interest continue to accrue.” An IDRS-monitored agreement generates a CP521 before each payment date unless the client pays by direct debit. The notice stops only when the balance is paid, the agreement is converted to a direct debit agreement, or the agreement defaults and the account leaves status 60.
The IRM notes a few features of the notice cycle that explain odd timing. The campus that issues the CP521 is set by the client's business operating division and state when the agreement is set up, and IRM 5.19.1 gives the example of a Fresno-monitored agreement that stays at Fresno even after the taxpayer adds a Schedule C. Payments mailed with the stub go to a lockbox bank; IRM 5.19.1 says “Lockbox will only process payments with the CP 521, Installment Agreement Reminder Notice, or CP 523, Installment Agreement Default Notice,” so a payment sent to a lockbox without the stub is misrouted.
Deadlines and what happens if they pass
The due date on the CP521 is the agreement's payment date. IRM 5.19.1.6.4 instructs employees to “Advise the taxpayer to mail payments at least ten days in advance of the due date, even if CP 521, Installment Agreement Reminder Notice, is not received.” A missing reminder is not an excuse for a missed payment.
| Event | What the IRM says | Source |
|---|---|---|
| First missed payment | A systemic skip is allowed on most agreements; the account stays in status 60 | IRM 5.19.1.6.4.19 (missed payments are “usually the skipped payment and a second missed payment”) |
| Second missed payment | Account moves to status 64 and a CP523 default notice issues | IRM 5.14.11.4 |
| More than two skips in 12 months | Agreement is defaulted by CSCO unless a new financial statement is provided | IRM 5.14.11.5 |
| New liability not paid | Grounds for default even if every installment was paid | IRM 5.14.11.3; IRC 6159(b)(4)(B) |
IRC 6159(b)(4) lets the IRS alter, modify or terminate an agreement when the taxpayer fails “to pay any installment at the time such installment payment is due under such agreement,” fails “to pay any other tax liability at the time such liability is due,” or fails “to provide a financial condition update as requested by the Secretary.” IRM 5.14.11.3 adds that the new-liability rule “includes related TINs for the same taxpayer,” such as a sole proprietor's employment tax account, and closes with: “Installment agreements may not be defaulted nor terminated for reasons other than those listed in this section.” Before any of that happens the law requires 30 days' written notice with an explanation (IRC 6159(b)(5)); that notice is the CP523.
IRM 5.14.11.5: “If a taxpayer skips more than two payments in a twelve-month period, including the systemic skip, the agreement will be defaulted by CSCO unless the taxpayer provides a new or revised financial statement.” Re-evaluation can be done by phone, fax or correspondence; the taxpayer does not have to appear in person.
How to respond
- Pay the amount shown by the due date, using the stub and envelope if paying by mail, or through IRS Direct Pay or the client's Online Account. The IRS page says to include the bottom part of the notice so the payment is credited correctly.
- If the client cannot make this payment, call the number on the notice before the due date. IRM 5.19.1.6.4.19 shows that a taxpayer who “made the required payment after CP 523” issuance is reinstated, but a call before default avoids the reinstatement fee and the default record entirely.
- If the client's circumstances have changed, request a revision. Under IRM 5.19.1.6.4.19 an account that meets Simple Payment Plan criteria can be revised with “No managerial approval,” “No NFTL determination” and “No financial statement”; above those limits the IRS secures a Form 433-F. See installment agreements and the eligibility calculator.
- Consider converting to a direct debit agreement. IRM 5.19.1.6.4 says taxpayers who enter DDIAs “are subject to lower user fees,” the systemic skip issue goes away, and the CP521 stops.
- Watch the current year. A balance on the next return that is not paid in full is a default under IRM 5.14.11.3 even when every installment was made. Adjust withholding or estimates now; IRM 5.14.11.5 lists “if the taxpayer is in compliance with estimated tax requirements and/or has adequate withholding” among the reinstatement considerations.
- Check the posting. The account transcript should show each payment (TC 670) against the intended period. A misapplied payment is grounds to reverse a default: IRM 5.19.1.6.4.19 treats an agreement that “defaulted due to IRS error” as one to restore at the previous terms with the reinstatement fee waived.
What the IRS does next
While the agreement is in effect, the account carries a TC 971 AC 063. IRM 5.19.1.6.4 states the protection: “No levies may be served while an IA is pending, in effect, or for 30 days following rejection or termination and the appeal period if the rejection or termination is timely appealed.” The failure to pay penalty on a qualifying individual account is cut in half: “Failure to Pay penalty is reduced to 1/4% per month for all accounts in an IA status which meet the criteria set forth in IRC 6651(h),” and the rate is reduced again when a defaulted agreement is reinstated (IRM 5.19.1.6.4.6). See the failure to pay penalty.
User fees are charged once at origination and again at reinstatement or restructuring. For agreements from July 1, 2024 onward, IRM 5.19.1.6.4.6 lists: $178 for a regular or payroll deduction agreement set up manually, $107 for a direct debit agreement, $69 and $22 for the same agreements set up through the Online Payment Agreement application, $43 for low-income taxpayers (reimbursable in some cases) and $0 for a low-income direct debit agreement. Reinstatement or restructuring is $89 manually or $10 online, with the $43 and $0 low-income rates. The IRM notes that the Bipartisan Budget Act of 2018 capped the fees at their February 9, 2018 levels, which is now IRC 6159(f).
If a payment is missed and the skip is used up, the account moves to status 64 and the CP523 issues by certified mail with a 30-day termination date and CAP appeal rights. If the agreement terminates, the account returns to active collection (status 22 or 26) and the levy protections of IRM 5.19.1.6.4 end once the post-termination period runs.
Related notices
- CP523: the default notice and intent to terminate the agreement.
- CP14: a new balance due on another period, which can itself default the agreement if unpaid.
- CP71: the annual reminder of the balance, sent even while the agreement is current.
- CP22A: an adjustment that changes the balance under the agreement.
Questions practitioners ask
The client did not receive a CP521 this month. Is the payment still due?
Yes. IRM 5.19.1.6.4 tells employees to advise that payments be mailed at least ten days before the due date even if the reminder is not received. The campus payment address without the stub is available from the IRS by phone; the IRM says not to give out lockbox addresses except on the notice.
Does one missed payment default the agreement?
Usually not. IRM 5.19.1.6.4.19 describes a default as typically the skipped payment plus a second missed payment, and IRM 5.14.11.5 defaults an agreement when more than two payments are skipped in twelve months. A new unpaid liability is a separate default ground.
Can the payment amount be lowered?
Yes, by revision. Below the Simple Payment Plan thresholds the IRM requires no financial statement or managerial approval; above them, the IRS will ask for Form 433-F or a comparable statement and reviews ability to pay.
Is interest still running?
Yes. IRM 5.19.1.6.4 says penalties and interest continue to accrue during an installment agreement. The failure to pay rate is reduced to one quarter percent per month for individuals who filed on time (IRC 6651(h)); interest is not reduced.
Does a CP521 extend the collection statute?
No. The agreement itself does not extend the CSED, though time while a request is pending and certain periods after rejection or termination are added. See the collection statute page.