Certain conditions must be met before an attic truss can be built.
Build attic chief architect.
The bottom line is this.
To modify the area above the garage using the select objects tool click in the area or room that is above the garage on the 2nd floor to select it and click the open object edit button.
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.
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.
You can build a cape cod or story and a half house by creating a new floor and rebuilding the roof.
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.
To manually add a gable wall.
If you have a half story structure you must learn to live without the auto roof build function once you have built the roof.
With pro you can lock certain roof planes and perhaps leave auto roof on but it is best not to rely on that.
Repeat this process for the opposing gable wall on this floor.