DavidJames

Go to your foundation level, and open up your room specification dialogue box for the stairwell.

Once there, select structure and then change your ceiling height to "-4". I'm suspecting yours is probably "4". 


 

If you draw a polyline rectange and outline the opening, you can convert the rectangle to a terrain feature. Once converted, open up the properties and change it to a "hole in terrain". 


As for the walls being invisible, are you referring to them not being seen in the floor plan? If so, that's just your visibility settings. 

You can do this by using the rectangular polyline tool, outlining your dormer, and then converting it to a "Hole in Roof / Custom Ceiling". 

Once converted, it'll actually create two separate polylines. One for your main roof, and the other for your dormer. Simply delete the polyline for the dormer. 

Within the hole properties, you can select your cut style preference (square, plumb etc).

On something completely unrelated, what hatch pattern is that you are using for stucco?