Write the apartment number on its own line, after the street address

The standard format places your apartment number on a separate line between your street address and your city, state, and ZIP code. The most common way to write it is Apt. 302 or #302, though Unit 302 also works. The key is that it comes after the building address, not before it.

Here is what a complete address looks like:

Sarah Chen 450 Oak Street Apt. 302 Denver, CO 80202

The apartment number tells the mail carrier or delivery person which unit inside the building is yours. Without it, mail may sit in the building lobby, get delivered to the wrong unit, or be returned to the sender. For legal documents — leases, court notices, rental information applications — a missing apartment number can mean the document never reaches you at all.

Key Takeaways

  • Place your apartment number on its own line after the street address, using Apt., Unit, or # followed by the number.
  • For mail and packages, the apartment number prevents delivery to the wrong unit or to the building lobby.
  • For legal documents like eviction notices or lease agreements, a missing apartment number can delay or prevent delivery.
  • If your building uses letters instead of numbers (Apt. A, Apt. 3B), write it exactly as shown on your lease or mailbox.
  • When filling out online forms, use the apartment field if one exists; do not add it to the street address line.

Why the apartment number matters for legal documents

Landlords, courts, and government agencies use your full address — including apartment number — to serve you with official papers. An eviction notice, a lease, or a notice to pay rent must reach the correct unit. If the address is incomplete, the process may be delayed, or the document may be deemed delivered even if you never received it, depending on your state's rules.

For rental information programs, housing authority applications, and tenant dispute filings, the apartment number ensures your file is matched to the right person. Many buildings have multiple units, and a missing number creates confusion that can slow down your case.

Different formats and which ones work

The U.S. Postal Service accepts several formats. The most widely recognized are:

  • Apt. 302 — the traditional abbreviation
  • #302 — common in informal writing and some regions
  • Unit 302 — increasingly standard, especially for newer buildings
  • Apartment 302 — the full word, always correct but less common

If your building uses a letter or combination (Apt. 3B, Unit 12C), write it exactly as it appears on your lease, mailbox, or building directory. Some older buildings or converted houses use letters instead of numbers. Consistency matters more than which format you choose — use the same format every time you write your address.

How to fill in apartment number on online forms

Many websites and applications have a separate field for apartment or unit number. If the form has one, use it. Do not add your apartment number to the street address line — that can confuse the system and cause mail or documents to be returned.

If the form does not have an apartment field and only shows "Street Address," put your apartment number on a second line within that field if the box allows it. Some forms will have a second address line labeled "Address Line 2" — that is where your apartment number goes. If there is no second line and no apartment field, write it on the same line as the street address, after the number: 450 Oak Street Apt. 302.

What to do if your building has no apartment number

Some smaller buildings, duplexes, or converted houses do not use apartment numbers. Instead, they may use directional labels (Front, Back, Upper, Lower) or no designation at all. If your lease or mailbox does not show a number or unit designation, write your address with the street number and street name only.

If mail or documents are being delivered to the wrong place, contact your landlord or building manager to ask what designation the building uses. They may have assigned one that is not obvious from the outside. If the building truly has no system, ask your landlord to clarify in writing what address to use for official documents.

Apartment numbers on legal documents and court filings

When signing a lease, make sure the apartment number is written on the document. When filing a complaint with a housing authority, submitting a rental information process, or responding to a court notice, include your full address with the apartment number. Courts and agencies use this to match documents to the correct case file.

If you move within the same building to a different unit, update your address with your landlord, your employer, your bank, and any government programs you are enrolled in. A change in apartment number is a change of address, even if the street address stays the same.

Common mistakes that delay mail and documents

Leaving off the apartment number entirely is the most common error. Mail carriers may leave it in the lobby or return it to sender. Writing the apartment number on the same line as the street address without proper spacing or punctuation can confuse automated sorting machines. Using an outdated apartment number after you move, or using a number that does not match your lease, creates a mismatch that delays delivery.

Another mistake is writing the apartment number before the street address. The correct order is always: name, street address, apartment number, city, state, ZIP code. Reversing this order can cause the mail system to reject or misroute the piece.

Frequently Asked Questions

Do I need to write "Apt." or can I just use the number?

You can use Apt., Unit, #, or the full word Apartment — all are accepted by the postal service. The number alone (just "302") is less clear and may confuse mail carriers or automated systems. Using a prefix like Apt. or Unit makes it obvious that 302 is an apartment designation, not a street number.

What if my apartment number has a letter, like 3B?

Write it exactly as it appears on your lease and mailbox: Apt. 3B, Unit 3B, or #3B. The letter is part of your address and must be included every time. If you leave it off, mail may go to Apt. 3 instead, which is a different unit.

Should I put my apartment number in the street address line on online forms?

No. If the form has a separate field for apartment or unit number, use that field. If it has an "Address Line 2" field, put your apartment number there. Only add it to the street address line if the form has no other option and allows multiple lines within a single address field.

What happens if I forget my apartment number on a legal document?

The document may still be delivered, but there is a risk it goes to the wrong unit or sits in the lobby. For important papers like a lease or court notice, always include your full address with the apartment number. If you realize you omitted it, contact the sender and provide the correct address when ready.

Can I use a PO box instead of my apartment number?

A PO box is a separate mailing address and is not a substitute for your physical apartment address. For legal documents, court filings, and official notices, you must provide your actual apartment address. You can use a PO box for mail you choose to receive there, but your lease and legal documents should list your physical address.