This is one Biggest secret I want to tell you.
I have many Experience for Wall detailing and every time I want to manage the height Extension for wall I was in the biggest challenge .
The base method was unbelievable tough, you will go to the Edit Structure Dialogue and make unlock the top or bottom for finishing Layer and then change the Height for Top Extension and Bottom Extension Manually . I had many other limitation to , for example I cant change the height for interior and exterior layer individually and if I want to have the height extension with slop that wasn’t possible.
Now , you can see how much is easy to do this with this method .
As you see in the below picture we have an Toilet and many walls around the toilet and we have ceiling in the Toilet Area.
Select all of the walls around the toilet and hit the Attach icon.
Now you can see what happen for your wall. Pay attention I create the ceiling with extend to core option.
Make some undo , select the Ceiling and select Edit Boundary .
Create free Slope arrow and change the Height for tail and head to Zero.
Select Finish Button and select the walls and hit Attach Icon.
Now you can see what happen for your Wall finishing Layer. you can attach exterior layer and core boundary layer separately too.