Add a dormer to a roof, and use the Dormer Opening tool to cut the opening for it.
This video demonstrates the following:
When creating a dormer, if you place the walls and the roof for the dormer first, you can use these elements to cut the dormer opening in the main roof.
In this example, we will add a dormer to the roof in the master bedroom. First, we will place the 3 walls of the dormer. Select the Wall tool, and choose a wall from the Type Selector. On the Properties palette, set the Top Constraint for the walls to Unconnected, and specify a height of 4000. Place the 3 walls in the plan view. Next, the walls need to be moved to the roof line to create the dormer. Select the walls, and on the Properties palette, set the Base Constraint to Roof Line. You can see in the 3D view that this constraint moves the dormer walls up to the main roof.
Create the roof of the dormer using the walls we just placed. Select the Roof tool. On the Properties palette, set the Base Constraint to Roof Line and enter an offset of 2000 and an Overhang of 100. These values position the dormer roof above the main roof. Use the Pick Walls tool to create 3 of the roof boundary edges. Use the Line Segment tool to create the last edge. After the sketch is complete, finish the roof. In the dialog, click Yes to attach the dormer walls to the roof.
Now, use the Roof Join tool to join the dormer roof to the main roof. On the Modify tab, click Join/Unjoin Roof. In the 3D view, select one of the back edges of the dormer roof, and then select the plane of the main roof. The dormer roof is extended to meet the main roof. Use the Edit Profile tool on the dormer walls to adjust the base of the walls to meet the roof. Use the Pick Lines tool when editing the profile to match the top edge of the main roof.
Create the opening in the main roof using the Dormer Opening tool. Select the main roof as the element to be cut by the opening, then select the dormer roof and walls to create the sketch of the opening. In this case, we pick the inside edges of the side walls, and then use the section view to select the inside edge of the front wall. The inside edges are used so the walls of the dormer are on top of the main roof. Use the modify tools to close the sketch and click the check mark to finish and create the dormer opening.