Lowerin ceiling seight in Rooms without lowering wall height

[cocoabill]

I am trying to lower the ceiling in two rooms.  All walls in home are 120" and I want ceiling in entry and kitchen to be 109".  I have read everything I could find and as close as I can get is using shelf ceiling and using soffit for ceiling.  Almost works but gives me a gap between two ceiling heights.  Any suggestions?

I tried shelf ceiling but you get a gap between old ceiling and new, I tried just lowering the ceiling but that lowers the roof.  If I change roof back it raises the floor.   Any suggestions?

[Garybills]

I did these in Home Designer Pro 2021. Before lowering the ceiling I turned off Auto Build roof and the Ceiling dropped but the walls and Roof remained the same. 

[Rookie65]

Another option is to use a soffit surface for the ceiling at the thickness you need it to be. You can then set it to be placed under the ceiling. That way you can leave your auto build roof setting on in case you need to rebuild something down the road.

[MississippiMims]

The easiest way I've found is to draw a single ceiling plane over the room you want to lower.  Select the plane once it's drawn to open the dialogue.  Set the slope to 0 degrees and lock the slope.  Then set your ceiling height and apply.