How does American English distinguish between sharing a flat and a room?

Solution 1:

You're right. In American English, the term "roommate" only means that two unrelated people share the same address, the same residence, not the same room.

Here are some other related words that you would hear in America:

bunkmate: a person who shares the same sleeping quarters (oddly, people who share the same room still more often call each other roommate rather than bunkmate)(bunkmate tends to refer to either temporary sleeping quarters or the military)

housemate: a person who shares the same house (used for referring to living in houses, not apartments or flats)

cellmate: a person who shares the same prison cell

suitemate: a person with whom ones shares a suite, as in a college dorm; roommate; also, a person with whom ones shares an office suite

wallmate: a person who shares the same residence whose room is directly adjacent but not part of the same suite. (often used to describe dormitory living where two adjacent rooms don't share a single bathroom between)

flatmate: British. Americans don't tend to use this term since Americans don't tend to call apartments "flats." Americans don't commonly say "apartmentmate."

Solution 2:

In the US, it's very rare that outside of dormitories and children sharing a room with siblings, that people sleep together in the same room without being romantic or sexual partners. However, it is quite common to refer to two people sharing an apartment in separate rooms as apartment mates.

From the University of Indiana South Bend website:

-https://www.iusb.edu/housing/resources_forms/Housing%20Apartment-Mate%20Agreement.pdf

This Apartment-mate Agreement allows you and your apartment-mates to discuss potential issues and develop compromises for the common space in your apartment.

From the North Carolina State University website:

https://housing.ncsu.edu/roommate-agreement

Steps to Resolve Roommate Conflict 1.Speak to your roommate/suitemate/apartment mate directly. Review and discuss the Roommate/Suite/Apartment Agreement that was completed. State issues neutrally. Relate/discuss feelings. Offer resolutions. Be prepared to listen and to compromise.

And from Urban Dictionary:

apartment mate people living within the apartment but not in your same room.