I would suggest that you request to draw this line in a clone of the current home that will display only walls. You could display this clone in your dialog box with your own plan component / controller, using subclasses of PlanComponent and PlanController classes (VideoPanel class uses for example a subclass of PlanComponent).
Thanks for the suggestion :) I think this approach could be used in a future version. One of my main goals is, to store the roof-model (the gabels and the properties, so you can recalculate the roof after adding some jutty) so it would 'survive' to be opened and saved again by a blank SH3D (no plugins). i didn't get familiar with the saving process so i'll follow the first approach for my MVP.
I currently created custom components for my roof, and i do have a dormer that "cuts" half of the roof to create space for a big door window.
This dormer has a gabel. So just add the gabel.To join Roofs is one of my main goals... so 'checked'
I would also point out that usually (at least here where i live) roofs extends over the walls and covers also the outside!
i called this feature overhang. It will be part of the MVP