I couldn't resist this plan.
I think dormer framing would take up precious floor space, so I would add it as a regular "room", and then add the false soffit/fascia, so it still has the "dormer" look.
Although you have so many more options and tools in Chief, as opposed to Home Designer...but maybe this old post on hometalk will help jump start you.
https://hometalk.chiefarchitect.com/index.php?/topic/994-cant-get-this-roof-right-for-the-life-of-me/page-3