Can't Extend 1st Floor Interior Wall When Adding 2nd Floor

[Mike M]

Greetings,

I am planning a remodel of adding a 2nd floor over part of the 1st floor. I am having difficulty in keeping the interior wall correct when creating the 2nd floor. The two attached screenshots illustrate my problem:



I want to extend the first floor interior to match the pitch of the roof/ceiling line and eliminate the 2nd exterior wall that appears in the room.

This wouldn't appear to be an uncommon design feature but I cannot find anything in the forum.

Any help is greatly appreciated!