arthur17crusader

I have also tried a sloped-ceiling octagon but I didn't like the outcome, so I haven't got any pictures to show.

Secondly, yes, this is the ground floor and above that there's another floor. This was never a problem since I always built the rough ceiling taller than the finished ceiling and the ceiling finish is quite thick, so that the finished floor is 2.7m 

@ dshall: what is SOL? Am I the Sun? :-)

I've done this.
Two rooms, one within another, linked with an invisible wall. The inner room is made taller than the outer ( let's say 20cm taller) and all its walls are made invisible as well. At the exact same position of the invisible walls I put two soffits for each wall, one soffit on top of the other. The lower is a few mm thin to pretend the ceiling and of the same paint and the top reaches to the taller ceiling of the inner room, decorated with moldings. Once I deleted the "inner" ceiling to make it vaulted but I didn't like the outcome.

(Off topic: the opening on the left would receive a fitted cupboard/bookcase)