How to refer to an apartment on a specific floor?

Suppose that on the first floor of a building, there are three separate apartments numbered 1, 2 and 3 respectively. How can I refer to one of them when writing a postal address? I am wondering if there is any need to mention words like "apartment" or "unit" before the apartment number. Which one is more appropriate to use?

"Apartment 2, first floor"
"Unit 2, first floor"
"2, first floor"
"2 first floor"
"2 1st floor"

I also found this entry on Wikipedia which suggests that the words "lot" and "unit" can be used before the apartment number in Malaysia. (My question is more about American English though):

Salutation, Name of recipient
Name of Position & Department (if applicable)
Company name
Unit/Lot number, Building name/Commercial area
Lot number (for building), Street name
Postcode Post office/Mail centre
State (optional)
Country

example:

Lot 323, 1st Floor, Bintang Commercial Centre

Any advice would be appreciated.

Solution 1:

If you haven't received an address, then any of those should be understood, not seem rude, and get the message to its intended recipient, which is after all the entire point of the exercise.

If you've got an address from someone, then use the form they gave no matter how far from the formats you give seems; it might relate to some preference within the building or of the addressee. Just as with a name, you should use what the recipient prefers, even if it seems strange or silly to you.

(Your fourth does have the minor disadvantage of lacking a comma to help mentally space the number from the floor).

Solution 2:

If you're looking for the official answer, the USPS lays out standards for addressing unit numbers.

123 Main St Apt 2

City ST 12345

If you're not exactly sure, you can use this tool to enter as much information as you have, and it will convert it to the correct format. (Example)

Solution 3:

In American English, for simply a residential apartment, you'd likely use a number sign, like this:

My Friend Flicka
#3, First Floor
87654 First Avenue
City, ST Zip+4

Often though, the apartment follows the street address, like this:

My Friend Flicka
87654 First Avenue #3
City, ST Zip+4

My Friend Flicka
87654 First Avenue Apt. 3
City, ST Zip+4

And occasionally but infrequently immediately follows the street address, but this seems to usually be when the apartment is a letter:

My Friend Flicka
87654-B First Avenue
City, ST Zip+4

"Lot" is generally not used except for trailer park or industrial/business park addresses. "Unit" is used and is recognizable, in particular for business addresses and apartment complexes with multiple buildings.

"Suite" is also sometimes used, but generally for business addresses.