A checkbox would just be to turn the options on and off. Checkbox for a gutter, checkbox for fascias, etc. Once it's turned on, then the user could customize each. A different 3d program that I had been working in automatically made the fascia, gutter, and soffit for it's roof tool. For example, when I drew the roof with the manual tool, it made a 12" overhang based on pre-set settings, it placed a gutter (which could be turned off with a checkbox), and it placed the fascia based on pre-set figures. In Archicad, I'm having to go through a lot more steps on the roof tool to get the same things done that were automatic in the other program. The other program, however, could not do custom guttering or fascias. Unless I'm doing it wrong, right now I'm having to draw a slab for the fascia and trim back the roofs to meet the fascia. For instance, when I make a roof that 9 1/4" (for 2x10's), it makes a very large fascia area. I make a 5 1/2" deep slab (for a 1x6 fascia), and trim the roof back. Having the options that I recommended would speed things up a lot!
Thanks for the good vote, though.