Apartment, Unit, and Suite Address Formatting Guide (2025)
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Send Mail OnlineOne of the most common causes of delayed or returned mail is incorrectly formatted apartment, unit, or suite numbers. USPS has specific guidelines for "secondary address" elements—often called "address line 2." This guide covers the correct apartment address format, suite address format, and unit address format, showing you exactly how to write apartment numbers in addresses for proper USPS mail processing.
📋 Table of Contents
Address Formatting Basics
A complete address with secondary information (apartment, suite, etc.) follows this format:
📋 Standard Format
RECIPIENT NAME
STREET NUMBER STREET NAME APT/STE/UNIT #
CITY STATE ZIP
✅ Key Rules
- • Same line preferred: Put secondary info on street address line
- • Use abbreviations: APT, STE, UNIT, FL, BLDG
- • ALL CAPS: For best machine readability
- • No punctuation: Skip commas and periods
- • One space: Between abbreviation and number
❌ What NOT to Do
- • Put apartment number BELOW city/state line
- • Write out full words (Apartment, Suite)
- • Use punctuation (Apt. #4, Suite 200,)
- • Forget the apartment number entirely
- • Use inconsistent formatting
Standard USPS Abbreviations
Use these official USPS abbreviations for secondary address designators:
| Full Name | Abbreviation | Common Use |
|---|---|---|
| Apartment | APT | Residential apartments |
| Suite | STE | Offices, businesses |
| Unit | UNIT | Condos, townhomes |
| Floor | FL | Specific floor in building |
| Building | BLDG | Multi-building complexes |
| Room | RM | Hotels, dorms, offices |
| Department | DEPT | Large organizations |
| Space | SPC | Mobile homes, storage |
| Lot | LOT | Mobile home parks |
| # (Number) | # | Generic number indicator |
💡 Which Designator to Use?
Use whatever designation the building or complex uses. If the building calls it "Unit 5," use UNIT 5. If they call it "Apt 5," use APT 5. USPS prefers the standardized abbreviations (APT, STE, UNIT) over the # symbol. The # symbol works as a universal fallback when you're unsure, but specific designators are clearer for mail processing.
Where to Put the Apartment Number
USPS has specific rules about where secondary address information should appear:
✅ Preferred: Same Line as Street Address
JOHN SMITH
123 MAIN STREET APT 4B
NEW YORK NY 10001
This is the USPS-preferred format for best processing.
✅ Acceptable: Line ABOVE Street Address
JOHN SMITH
APT 4B
123 MAIN STREET
NEW YORK NY 10001
Use this only if the address is too long for one line.
❌ WRONG: Line Below Street Address
JOHN SMITH
123 MAIN STREET
APT 4B ← Wrong position!
NEW YORK NY 10001
This can cause processing errors. The apartment may be ignored.
Correct Examples by Type
Here are correctly formatted examples for different situations:
Apartment
JANE DOE
456 OAK AVENUE APT 12
CHICAGO IL 60601
Suite (Business)
ACME CORPORATION
ATTN JOHN SMITH
100 CORPORATE BLVD STE 500
DALLAS TX 75201
Unit (Condo/Townhome)
MIKE JOHNSON
789 MAPLE COURT UNIT 3
SEATTLE WA 98101
Building + Apartment
SARAH WILLIAMS
200 PARK PLACE BLDG C APT 301
BOSTON MA 02101
Floor Only
ABC COMPANY
555 BUSINESS CENTER FL 12
LOS ANGELES CA 90001
Mobile Home
BOB MILLER
123 TRAILER PARK RD LOT 45
PHOENIX AZ 85001
Common Mistakes to Avoid
These formatting errors can cause mail delays or returns:
| ❌ Wrong | ✅ Correct | Why |
|---|---|---|
| 123 Main St. Apt. #4 | 123 MAIN ST APT 4 | Same line, no punctuation |
| 123 Main Street Apartment Four | 123 MAIN ST APT 4 | Use abbreviations and numbers |
| 123 Main St New York NY 10001 Apt 4 | 123 MAIN ST APT 4 NEW YORK NY 10001 | Apt goes BEFORE city line |
| 123 Main St, Suite 5 | 123 MAIN ST STE 5 | No comma needed |
| 123 Main St # | 123 MAIN ST APT 4 | Include the actual number! |
🚫 The Biggest Mistake
Forgetting the apartment number entirely. Mail sent to multi-unit buildings without an apartment number may be returned, delayed, or delivered to the wrong person. Always include it!
Special Cases
Here's how to handle less common situations:
Letter + Number Combinations (APT 4B)
Include both the number and letter without spaces: APT 4B, STE 100A, UNIT 12C. This is common in large buildings.
Multiple Designators (Building + Apt)
When you need both building and apartment: BLDG C APT 301. Put them in order from largest to smallest area on the same line.
Very Long Addresses
If the address won't fit on one line, put the secondary designator on the line ABOVE the street address, not below. The city/state/ZIP always goes last.
PO Box + Street Address
If someone has both, use the delivery address they prefer. The line directly above the city/state is where USPS will attempt delivery. See our PO Box addressing guide.
Private Mailbox (PMB)
Private mailboxes at UPS Store, etc. use PMB or #: 123 MAIN ST PMB 456. Some also accept STE, but PMB is more accurate.
Certified Mail to Apartments
When sending certified mail to an apartment address, the same formatting rules apply. The apartment number goes on the same line as the street address, and you'll attach the certified mail label (PS Form 3800) above the delivery address. See our certified mail guide for details.
Frequently Asked Questions
Where does the apartment number go in an address?
The apartment number should go on the same line as the street address, after the street name. Example: 123 MAIN STREET APT 4B. If it doesn't fit, put it on the line directly above the street address, never below the city/state line.
What is the correct abbreviation for apartment?
The USPS standard abbreviation for apartment is APT. USPS prefers standardized abbreviations like APT, STE, or UNIT followed by the number for proper apartment address format. The # symbol (e.g., #4) is a commonly used alternative but serves as more of a fallback. Avoid writing out "Apartment" in full to keep addresses concise and machine-readable.
What's the difference between APT, UNIT, and STE?
APT (Apartment) is used for residential apartments and is the standard apartment address format. UNIT is the preferred unit address format for condos, townhomes, or multi-unit buildings. STE (Suite) is the correct suite address format for business or office addresses. Use whatever designation the building uses officially.
Should apartment numbers go on a separate line?
USPS prefers the apartment number on the same line as the street address (e.g., "123 MAIN ST APT 4"). If it doesn't fit, put it on the line ABOVE the street address, not below. Never put it after the city/state/ZIP line.
Can I use # instead of APT?
Yes, using #4 instead of APT 4 is acceptable. However, APT, STE, or UNIT followed by the number is the standard USPS format and is clearer for mail processing. The # symbol is a universal fallback.
What happens if I forget the apartment number?
Mail without an apartment number may be delayed, returned to sender, or delivered to the wrong recipient. For multi-unit buildings, the apartment number is essential for accurate delivery. Always double-check before mailing.
Is it okay to write the apartment number on address line 2?
USPS prefers the apartment number on the same line as the street address (e.g., "123 MAIN ST APT 4"). If you must use a second line, place it ABOVE the street address line—not below it. Placing secondary address information below the street address or after the city/state/ZIP can cause delivery issues because USPS systems read the line directly above the city as the delivery point.
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