Sitting room above garage

[ortmajc]

I am having a problem creating a sitting room that comes off my bedroom. I have tried using some of the tutorials but I seem to continue getting the wrong look. The sitting room is actually in the roof above the garage. Below I have attached a picture of my home, the floor layout, and the result that I am getting. What am I doing wrong? I am using Home Designer 8.0. I ran an exterier wall where the interier wall was and just created a gable roof off of the garage and it looked right from the outside, but then I couldn't create the room in the inside. The last picture is an image of that, which looks right, but I know I didn't do it the correct way. I just want to know what the proper way to do this is. Thanks.