If this is the case you will need to.
Build attic chief architect.
With pro you can lock certain roof planes and perhaps leave auto roof on but it is best not to rely on that.
The structure should be sufficiently wide to allow for a loft and attic spaces the loft area must be on a living floor and have attic rooms on both sides separated by knee walls a flat ceiling in the loft area and a floor below steep enough roof pitch to provide appropriate ceiling height for the loft area.
The bottom line is this.
Launch chief architect and open the plan in which you need to manually draw in the gable wall.
Certain conditions must be met before an attic truss can be built.
If the design calls for the roof to be built directly off of the 1st floor walls but retain a usable attic space you will need to generate roof planes that ignore the top floor.
To manually add a gable wall.
You can build a cape cod or story and a half house by creating a new floor and rebuilding the roof.
By chief architect.
On the structure panel of the room specification dialog that opens lower the finished ceiling value to the height that you want the exterior.
If you have a half story structure you must learn to live without the auto roof build function once you have built the roof.