Finally, I realized that in 2014 David Lynch had released Twin Peaks: The Missing Pieces. This film presented deleted scenes from the series and from Twin Peaks: Fire Walk with Me and guess what: camera looked down and revealed me stair knot :)
About the second floor: There aren’t even hints for half the rooms of the second floor. There is absolutely no information about the grey area in my watercolors. The creepy staircase and Laura’s room get almost all the attention of the camera.
I realized that the famous scene with the horse appearing in the Master bedroom wasn’t shot in the house itself, because who would manage to climb a horse to the second floor? Lynch built a special set resembling the bedroom. (Actually my grand grandfather rode a horse slowly to a second floor of a house and managed to get the horse back down the stairs which was actually the harder part. The horse freaks out from stairs, the noise of the hoof only ads to its horror, so grand grandfather (this happened during WWII in Macedonia) wrapped the hoof in soft cloth and sugar cube after sugar cube he rode the horse down the stairs and won a huge bet)