It may be a couple of years, but I have a dream of converting the roof of my 10x20 shed into an upstairs observatory. (sshhhh....my wife doesn't know yet!

) Is there a certain software app that most DIY'ers use to draw out their pre-planning drawings? (freebie preferred).
I'm not sure what type of roof I am going to use. Or if I am simply going to plop a SkyPod on a modified part of the roof. But I have seen many great ideas in this forum!
Thanks!
Art