Até onde sei, isso não é explicado em detalhes nos livros. Eu li todos os livros várias vezes e há poucas certezas:
The common room is on the 7th floor of Hogwarts behind the fat lady's painting
Agora, como explicado no primeiro livro, há duas escadas:
The stairs to the left lead to the boys dormitory, and the stairs to the right lead to the girl dormitory
Nada prova que existem vários andares para os dormitórios. No entanto, esta é uma citação do segundo livro:
"They [Harry and Ron] managed to get to the other side of the common room, still having their backs slapped, and gained the peace of the staircase. They hurried up it, right to the top, and at last reached the door of their old dormitory, which now had a sign on it saying SECOND YEARS. They entered the familiar, circular room, with its five four-posters hung with red velvet and its high, narrow windows. Their trunks had been brought up for them and stood at the ends of their beds."
-Harry Potter and the Chamber of Secrets.
E no quarto livro:
Harry, Ron, and Neville climbed up the last, spiral staircase until they reached their own dormitory, which was situated at the top of the tower.
-Harry Potter and the Goblet of Fire
Isto sugere que existem vários dormitórios acessíveis através das escadas em espiral em diferentes andares da torre.
Então, acho que sua imaginação e suposições estão certas. Espero que isso responda à sua pergunta.