One way to create something like this is to
1) Create the main gable roof
2) Comp split the roof face onto which you want to add the dormer part
3) Split the roof face to get the footprint for the dormer part
4) Extrude in the world.up direction to create the dormer part
I prefer to make a new leaf shape for objects like that and nudge them into place, like this:
Lot -->
House
RoofStructure
House -->
color(0,1,0)
extrude(10)
RoofStructure -->
color(1,0,0)
extrude(1)
s(1,11,1)
t(2,0,2)