Do you know if it's actually canon that Jim and Spock share a bathroom, or just very popular fanon?

This question is incredible because it made me search the internet for like twenty minutes trying to find blueprints of Enterprise deck plans which is not something I’ve ever been interested in

So, here’s the thing: this question is fundamentally unanswerable. I’m sorry. If we’re going from what we literally see in the show, the bathroom sharing is not canon and is purely a fanon invention since it was never mentioned in the series. In fact, according to this link right here, we don’t even know where Spock’s quarters are. And I quote: “only three people on the USS Enterprise have had their quarters mentioned in canon. James T. Kirk (Deck 5, Room 3F 121), Leonard “Bones” McCoy (Deck 9, Section 2, Room 3F 127), and Janice Rand (Deck 12, Room 3F 125 and later Deck 12, Room 3C 46) Spock’s quarters isn’t mentioned.”

Additionally, it is impossible to determine from blueprints, schematics, etc. where people were quartered. Deck plans of the NCC-1701 do not exist. I mean, at the time the show creators did NOT think any of this shit through, they thought people were going to be watching this show once a week and not thinking anything of it. They would stick random rooms all over the place with random numbers and not care or think about it at all without knowing that years later people would be writing fanfictions where actual concrete information about the ship’s layouts is important, or desperately wanting to know in excruciating detail every nook and cranny of the USS Enterprise. But based on this complete lack of information one way or the other, sure Kirk and Spock shared a bathroom!

However, deck plans of the Enterprise refit seen in the energetic and delightful piece of cinema “Star Trek: The Motion Picture” do exist. If we’re to assume that the Enterprise seen in the show has more or less the same deck plan as the TMP refit, I’m going to have to say no, it’s very unlikely that they shared a bathroom. Many of the rooms in the crew quarters do have shared bathrooms, as seen below: 

But there are two decks full of bigger and nicer quarters that include private bathrooms, seen below (sorry the pictures are cropped weirdly I’m lazy):

Now I would assume the captain and first officer are in the nicest available quarters, which means they probably do in fact have their own bathrooms.

But really we don’t know! And the older I get the more I feel comfortable with not knowing things and placing fanon above canon in terms of importance. Because all TV shows and books and movies are made-up stories! And stories are meant to be shared and interpreted and molded to suit people (in my opinion). They don’t necessarily hold any absolute truth. Really it’s better when things are left open to interpretation, because the way I fill in the blanks is just as valid as the way someone else might! And it’s fun to speculate!

So in a way Kirk and Spock sharing a bathroom is real because so many people have decided it’s real!

TLDR: The showrunners on TOS had no idea what they were getting into and didn’t plan out crew quarters, and I don’t think canon is necessarily this Pinnacle of Absolute Truth that most people consider it to be. 

-Spenser