Keep the unit on the street line, at the very end, with a standard designator: 1234 MARKET ST APT 5B. Publication 28 calls that the preferred position and only allows the unit to move up to the line above when the delivery line has run out of room.
The unit belongs at the end of the delivery line. Sending it to a second line is the habit that produces most corrections.
Every row is a situation people ask about, with the version the Postal Service address files are built around. Rules come from Publication 28 sections 213 and Appendix I at pe.usps.com, checked 2026-07-28.
| Situation | The version that causes trouble | The version to print |
|---|---|---|
| Apartment pushed to its own line | 1234 MARKET ST APT 5B | 1234 MARKET ST APT 5B |
| Suite spelled out | 88 PARK PLZ SUITE 300 | 88 PARK PLZ STE 300 |
| Pound sign with no space | 1234 MARKET ST #5B | 1234 MARKET ST # 5B |
| Delivery line genuinely too long | 12345 W CHESTERFIELD BLVD APT 1204 | APT 1204 12345 W CHESTERFIELD BLVD |
| Named building with no street number | COND VERDE APT 1120 | 1 COND VERDE APT 1120 |
| Several towers sharing a name | VISTA SUITES III APT 104 | 3 VISTA SUITES APT 104 |
Apartment deliveries mostly happen in dense city ZIPs, so we priced the run to one. This is a 12 x 10 x 6 in box at 5 lb from Newark to downtown Chicago, quoted through the SMKlog calculator on a single day.
| Carrier | Service | Price | Quoted transit |
|---|---|---|---|
| USPS | Ground Advantage | $14.02 | 8-9 business days |
| UPS | Ground Saver | $14.02 | window not quoted |
| UPS | Ground | $14.55 | 2-3 business days |
| UPS | 3 Day Select | $19.94 | 2-3 business days |
| UPS | 2nd Day Air | $21.96 | 2-3 business days |
Two rows tied at $14.02 with wildly different speeds, and fifty-three cents moved the estimate from 8 to 9 business days down to 2 to 3 on UPS Ground. Whichever row you pick, the unit number decides whether the last hundred feet happen at all.
Publication 28 lists the common ones as APARTMENT to APT, BUILDING to BLDG, FLOOR to FL, SUITE to STE, UNIT to UNIT, ROOM to RM and DEPARTMENT to DEPT, with the full approved set in Appendix C2. Section 213 states these designators "are required to be printed on the mailpiece for address locations containing secondary unit designators", and names the end of the delivery line as the preferred spot, per pe.usps.com, checked 2026-07-28.
Section 213.2 is specific: "If the pound sign (#) is used, there must be a space between the pound sign and the secondary number." Section 213 goes further and discourages the symbol outright when a real designator is known or already appears in the ZIP+4 file, so APT 5B beats # 5B whenever you know it is an apartment. A note attached to the same section warns that the symbol may be barred entirely on a commercial mail receiving agency address carrying private mailbox information.
Address checking works in two layers, and only the first one is reliably present. The Postal Service delivery point product can report that a known address has verifiable primary but not secondary data, meaning the street address matched and the unit did not, per postalpro.usps.com, checked 2026-07-28. Our checkout runs the destination against carrier address records before payment and stops the sale when it cannot be matched at all, but a real building with a blank unit line is a different failure, and the only fix for it is asking the recipient.
At the end of the street address line, after the street name and any directional. Publication 28 names that the preferred location for secondary unit designators and only moves them to the line above when the delivery line cannot hold everything, per pe.usps.com, checked 2026-07-28.
Use APT whenever you know it is an apartment. Section 213 discourages the pound sign as a designator when the correct one is known or already shown in the ZIP+4 file, and section 213.2 requires a space between the symbol and the number if you do use it.
The building can still match the address files while the unit does not, which is why a parcel can survive a check and then stall on the doorstep. From there the carrier either researches the address and bills a correction, or sends the box back to the return address.
On 2026-07-28 a 12 x 10 x 6 in box at 5 lb from Newark quoted $14.02 with USPS Ground Advantage and the same $14.02 on UPS Ground Saver, rising to $21.96 for UPS 2nd Day Air. Your own box and ZIP pair set the real figure, so quote it live.