How to Add Apartment Number After Mailing: USPS Fix for Missing Unit (2025)
Quick Answer: Can You Add an Apartment Number After Mailing?
Not directly on the package, but you have options:
- • Package Intercept: Redirect to corrected address (~$16–$20)
- • Contact recipient: Alert them to watch for it at the building
- • Hope for the best: Carrier may figure it out or it returns to you
Reduce the Chance of Missing Apartment Numbers
When you send mail through The Letter Pilot, our address verification helps catch missing apartment numbers and other common errors before your mail is sent.
Send Certified Mail OnlineRealizing you forgot to include the apartment number on a USPS package or letter you just mailed is a sinking feeling. Unfortunately, you can't physically add it to the label after it's in the mail stream. But don't panic—there are several ways to try to fix the situation, and in many cases, the mail still reaches its destination.
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What Happens to Mail Without an Apartment Number
When mail arrives at an apartment building without a unit number, several things can happen depending on the building size, the carrier's familiarity with residents, and the building's mail handling procedures.
Best Case: Carrier Delivers Correctly
In smaller buildings or when the carrier knows residents by name, they may be able to deliver the mail to the correct unit anyway. This is more likely if:
- • The recipient has an uncommon name
- • It's a small building (fewer than 10 units)
- • The carrier has served the route for a long time
Common Case: Held at Mailroom/Office
In apartment complexes with a building office or package room, mail without a unit number is often held there. The recipient may receive a notice or the office may contact them.
Returned as Insufficient Address
In larger buildings where the carrier can't determine the correct unit, mail is returned to sender marked "Insufficient Address". This typically takes 1-3 weeks, though times can vary.
Worst Case: Delivered to Wrong Unit
Occasionally, mail may be delivered to the wrong apartment, especially if another resident has a similar name. Recovery depends on the honesty of the wrong recipient.
Options to Fix Missing Apartment Number
Here are your options for trying to get the mail delivered correctly:
| Option | Cost | Success Rate | Speed |
|---|---|---|---|
| Package Intercept | ~$16–$20 | High (if fast) | Fast |
| Contact Recipient | Free | Varies | Fast |
| Wait for Return | Free | Likely | 1-3 weeks |
| Hope for the Best | Free | Varies | Normal |
Using Package Intercept
USPS Package Intercept allows you to redirect mail to a corrected address. This is your best option if you have a tracking number.
How to Use Package Intercept
- 1. Confirm your mail has tracking (Certified Mail, Priority Mail, etc.)
- 2. Go to USPS.com/intercept
- 3. Enter your tracking number
- 4. Choose "Redirect to New Address"
- 5. Enter the complete address WITH apartment number
- 6. Pay the fee (around $16–$20; check current USPS pricing)
Important Limitations
- • Tracking required: Regular First-Class letters without tracking can't be intercepted
- • Act fast: Must request before delivery
- • Not guaranteed: Package Intercept is "best effort"
- • Additional postage: May apply for redirect
For a complete guide, see How to Fix a Wrong Address After Mailing.
Contacting the Recipient
If Package Intercept isn't an option (no tracking, or you want to try a free solution first), contact the recipient directly.
What to Ask Them
- • Check their mailroom/building office: Mail without unit numbers often ends up there
- • Watch for delivery: It may come to their mailbox anyway
- • Ask their carrier: The recipient can ask their mail carrier to watch for it
- • Check with neighbors: It may be misdelivered to a nearby unit
If Tracking Shows "Delivered"
If tracking shows delivered but the recipient doesn't have it, they should:
- • Check mailroom/package room
- • Ask building management
- • Ask neighbors if it was misdelivered
- • Contact their local post office
What If It's Returned to Sender?
If your mail is returned marked "Insufficient Address," you'll receive it back with this endorsement stamped on the envelope.
What to Do When It Returns
- 1. Get the correct address with apartment number from the recipient
- 2. Prepare a new envelope with the complete address
- 3. Re-mail with the corrected address
- 4. Consider using certified mail for tracking on the resend
Can You Just Add the Apt Number and Re-Mail?
No. You cannot cross out the old address and write a new one on returned mail. You need to prepare a new envelope or package with proper postage and a correctly formatted address.
How to Prevent This in the Future
Best Practices
- • Ask for complete address: Always request apartment/unit numbers
- • Verify addresses: Use USPS Address Lookup
- • Double-check before sending: Read the label before dropping off
- • Save correct addresses: Keep an address book
Proper Apartment Address Format
John Smith
123 Main Street Apt 4B
Los Angeles, CA 90001
Always put the apartment number on the same line as the street address, or on a separate line immediately below.
Accepted Apartment Designators
Use Online Mailing With Address Verification
When you send mail through The Letter Pilot, we validate addresses before printing. Our system helps catch missing apartment numbers and other common errors before your mail goes out.
Send Certified Mail OnlineFrequently Asked Questions
Can I add an apartment number after mailing a package?
Not directly, but you have options. Use USPS Package Intercept to redirect the package, contact the recipient to watch for it, or wait for it to be returned as undeliverable if the carrier can't determine the correct unit.
What happens if I forget the apartment number on mail?
Without an apartment number, USPS may return the mail as "Insufficient Address," deliver it to a building office or mailroom, or in some cases the carrier may know the recipient and deliver correctly.
Will my package be delivered without an apartment number?
Sometimes. In small buildings, carriers may know residents by name and deliver correctly. In large apartment complexes, mail is often returned as undeliverable or held at the building's mailroom if one exists.
How do I prevent missing apartment numbers in the future?
Always verify addresses with recipients, use address validation tools like USPS Address Lookup, include "Apt", "Unit", or "#" before the number, and use online mailing services with built-in address verification.
Can USPS look up an apartment number?
USPS does not look up apartment numbers for individual mailings. You need to provide the complete address. The carrier may figure it out if they know the resident, but this isn't guaranteed.
Where does the apartment number go on an envelope?
The apartment number should go on the same line as the street address after the street name, or on the line directly below. Example: "123 Main St Apt 4B" or with Apt 4B on a second line.
Get It Right the First Time
Our address verification helps catch missing apartment numbers before your mail is sent, reducing the chances of returns and extra intercept fees.
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Tags: add apartment number, missing unit number, fix address after mailing, apartment address, USPS apartment