Secondary Address Unit Designators – Official List and FAQs
Firstlogic Team
In U.S. addresses, elements like “Apartment” or “Suite” are referred to as secondary address unit designators. Here we provide a list of these designators and their official USPS abbreviations.
Complete Table of USPS-Approved Secondary Unit Designators
| Designator | Abbrev. | Description | Requires Number? |
|---|---|---|---|
| Apartment | APT | Residential unit within a building. | Yes |
| Basement | BSMT | Basement-level unit. | No |
| Building | BLDG | Distinct building within a complex. | Yes |
| Department | DEPT | Department within an organization. | Yes |
| Floor | FL | Specific floor of a building. | Yes |
| Front | FRNT | Front section of a building. | No |
| Hangar | HNGR | Aircraft/large-equipment hangar. | Yes |
| Key | KEY | Key stop (internal delivery routing). | Yes |
| Lobby | LBBY | Lobby area. | No |
| Lot | LOT | Specific lot within a development/park. | Yes |
| Lower | LOWR | Lower portion of a building. | No |
| Office | OFC | Office unit. | No |
| Penthouse | PH | Penthouse unit (can be used without a number; if a building assigns a PH number, include it). | No |
| Pier | PIER | Pier address. | Yes |
| Rear | REAR | Rear section of a building. | No |
| Room | RM | Numbered/named room. | Yes |
| Side | SIDE | Side section of a building. | No |
| Slip | SLIP | Boat slip/docking space. | Yes |
| Space | SPC | Space within a complex (e.g., MH/RV parks). | Yes |
| Stop | STOP | Mail stop code (internal routing). | Yes |
| Suite | STE | Suite within a building. | Yes |
| Trailer | TRLR | Trailer lot or unit. | Yes |
| Unit | UNIT | General-purpose unit designator. | Yes |
| Upper | UPPR | Upper portion of a building. | No |
Groupings by USPS Requirements
The following tables show which designators must be followed by a number/identifier and which can stand alone.
Use this when validating inputs or normalizing addresses for CASS/carrier routing.
Group 1: Requires a Unit Number
| Designator | Abbrev. | Example |
|---|---|---|
| Apartment | APT | APT 5B |
| Building | BLDG | BLDG 3 |
| Department | DEPT | DEPT A |
| Floor | FL | FL 12 |
| Hangar | HNGR | HNGR 4 |
| Key | KEY | KEY 123 |
| Lot | LOT | LOT 27 |
| Pier | PIER | PIER 7 |
| Room | RM | RM 101 |
| Slip | SLIP | SLIP B12 |
| Space | SPC | SPC 42 |
| Stop | STOP | STOP 15 |
| Suite | STE | STE 210 |
| Trailer | TRLR | TRLR 8 |
| Unit | UNIT | UNIT 3A |
Group 2: Can Stand Alone
| Designator | Abbrev. | Notes |
|---|---|---|
| Basement | BSMT | Basement-level unit. |
| Front | FRNT | Front section of a building. |
| Lobby | LBBY | Lobby area. |
| Lower | LOWR | Lower portion of a building. |
| Office | OFC | Office unit (can be used without a number; if a building assigns an OFC number (e.g., OFC 2), include it). |
| Rear | REAR | Rear section of a building. |
| Side | SIDE | Side section of a building. |
| Upper | UPPR | Upper portion of a building. |
| Penthouse | PH | Can be used without a number. If building assigns a PC number (e.g., PH 2), include it. |
What are Secondary Address Unit Designators?
In U.S. addressing, elements like Apartment or Suite are formally referred to as secondary address unit designators. These identifiers are standardized by the United States Postal Service (USPS) in Publication 28 and are used by address validation software and mail carriers. A common, less official term for this address element is simply the secondary unit designator, unit designator, or unit description.
FAQs – Unit Designators and Secondary Addresses
Do I need to use USPS-standard abbreviations?
Yes. It is generally recommended to use the USPS-approved abbreviations (e.g., APT, STE, RM) from Publication 28. Non-standard forms like “Apartment No.” or “Suite #” are discouraged.
Can I use the pound sign (#) instead of a designator?
No. USPS discourages using the pound sign (#) when the proper secondary unit designator (such as APT or STE) is known.
What is a secondary address?
According to the USPS, the secondary address unit designator (e.g., STE) combined with its secondary address unit number (e.g., 210) is formally known as the secondary address.
Can a unit number contain letters?
Yes. Unit numbers may include letters as well as digits, such as “APT 2A” or “STE B”.
What happens if the unit number is missing?
Validation engines (such as USPS CASS or carrier routing systems) typically flag addresses as incomplete if designator (e.g., APT, STE) is missing its unit number. The only exceptions are certain designators that do not require unit numbers (e.g., LBBY, OFC).
Is the secondary address always stored in “Address Line 2”?
Not necessarily. While many databases and entry forms treat “Address Line 2” as the field for secondary addresses, this convention is ambiguous. That field may contain other data (such as building names or care-of lines), and secondary address elements are sometimes placed in “Address Line 1”.
About Firstlogic
Firstlogic specializes in self-managed solutions for address validation, geocoding, data quality, and data matching.
With tools like the Data Quality IQ Suite and Address IQ SDK, enterprises can validate addresses in real time, process data in batch, and maintain full control of their data within on-premise or private cloud environments.
Source: USPS Publication 28 – Postal Addressing Standards (Appendix C.2).
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