There's a few instances I've seen in the gemarra where there were meetings in an attic. Both that come to mind are with Rabbanan Gamliel, although I'm not sure if it occurs with others. One is the Mishnah that he had diagrams in his attic to test witnesses about the New Moon (Yachin there brings the following gemarra that this is where they accepted witnesses). The other is he formed a Beis Din in his attic to rule on the intercalation of the year (Rashi there says it was designated to be a location for a Beis Din)
I'm wondering why specifically an attic. Was that simply his office? Or was he hiding up there. I ask because I once heard that during the times of Chazal there was a decree against learning Torah, so they had to teach in hiding, namely in their attic. Is this true?
Edit: I found another example of something happening in an attic in Kesubos 50b, this time not involving Rabban Gamliel, rather a quote from an Amora.
דאמר רב יצחק בר יוסף בעלייה התקינו שיהו בנות ניזונות מן המטלטלין
As Rav Yitzchak bar Yosef said: When the Sages sat in the upper chamber to rule on certain halakhot, which they could not do in the study hall at that time due to persecution by gentiles, they instituted that daughters should be sustained from movable property in addition to land.
Note: the Sefaria translation mentions what I heard, that they had to hide in the attic due to persecutions. It's coming from Rabbi Steinzaltz commentary on that gemarra. I'm not posting this as an answer as I'd like an earlier source.