So I don't know how to describe, design, or find any accurate images of the Hogwarts Houses common rooms, so we're going to have to pretend that the Hogwarts Legacy common rooms are still the common rooms.

(Note Hogwarts Legacy takes place in 1890[I think, I don't have the game] and our story takes place in 1991-1998. You're just gonna have to pretend that the common rooms are magically cleaned/properly refurbished over the summer whenever something happens. [Example of when Dumbledore set the Gryffindor bed curtains on fire.] The doors open up the same, but to keep dignity to the books, there are certain ways you have to get the door to open. Gryffindor a password, Ravenclaw solving a riddle, etc..

Gryffindor:

Slytherin:

Ravenclaw:

Hufflepuff:

So it took a little bit for me to think it up, but I've got it.

Gryffindor
     •To get the Fat Lady's portrait to open, you have to be able to prove your bravery, or say the password(Which changes weekly). In cases where bravery can't be proven and you didn't catch the new password, you better hope the Fat Lady recognises you.

Slytherin
     •The door will show itself, but only unlock if you can find a cunning way to get it open(messing with the snake tail thing could possibly work), or say the password(changes every three days). Using Parseltounge is an easy substitute for the password, but incredibly unlikely among students because it is a rare ability. The door is magically charmed to not be affected by alohamora(I'm looking at you, Hermione).

Ravenclaw
    •The door is always visible, unlike the Slytherin and Hufflepuff* doors, but it is only accessible through answering a riddle to prove your intelligence; Or a password, in the unlikely case the Head of House decides there shall be one, but for the time being, there is not. For older students, particularly fourth-fifth years and above, the door is(was) charmed by Rowena Ravenclaw to open for any Ravenclaw who has been a student at Hogwarts for over four years, or her natural blood heir. (Though she has none living, due to the death of Helena Ravenclaw before she had any children.) The door is magically charmed to not be affected by alohamora(I'm STILL looking at you, Hermione). If you are unlucky enough to not be able to solve the riddle, HAHA! I mean, you better hope there's a someone who can solve the riddle if you're not a fourth year or above. The door, most of the time, will not open if you're not a Ravenclaw student.

Hufflepuff
    •The door is technically always visible, but a little harder to find due to it being identical to similar to other environmental items(aka, kegs/whatever those things are if they aren't wine/juice kegs). To get the door to actually open, you have to name a fact about any magical plant, animal,(ex. Devil's Snare, Mandrake; ex. Something about a unicorn I guess), or say the password. The password changes every two weeks, and a plant fact is used as an example to enter the common room at the beginning of the year. [1991 example of when Hogwarts's Whomping Willow was planted; 1971.]

As for the dorms, I went through this video and sorted through the time frames in which each dorm is visible.
For Ravenclaw, from 0:0 to 0:40
Hufflepuff, 2:57-2:59 ish to 3:22
Slytherin, 7:25 to 7:57
Gryffindor, 4:50 to 5:18
(I sorted them by thumbnail order)

PS. The time frames in which the password is changed depend on how well the House gets along with the others. Gryffindor, clocking in at a week, shows that they trust each House enough to go that long. Slytherin, every three days, shows that they are very suspicious of other Houses; Especially since they are considered the "evil" House. Hufflepuff, every two weeks, shows that they don't believe anyone would have any reason to get in their common room/dorms. Their password is not based on trust, but rather House superiority and dorm placement, since they reside in the basements. Ravenclaw, the House without a password, would change every week. Reasons similar to Slytherin(Trust issues).

Chapters (in book one) will say if you should check this incase you forget how the dorms look. After that, this will go mostly unmentioned.