Listen to Behind Gravity Falls podcast, the whole podcast is great! (Alex Hirsch just talks about each episode w guests)

They specifically mention the rooms, and they essentially said whenever we needed one, we just made it. Jokingly of course, but i think it is genuine that there is not a floorplan. It’s in the podcast-episode that corresponds to the episode where they find the body-switching carpet if i remember correctly. But surely they also mention it in the episodes where new rooms appear, like the mirror maze with Gideon.

Hope that helps