Thank you. On the attached image, the area above the red line must become a flat roof. Typically I've been able to comb through old posts and HD help to solve every challenge, but I cannot find a solution to this issue.
The back half of the house attic, including the area under the red line, will become a single room (dormers will be added).
Some context...this is an existing historic home that will have an addition on back right corner (as viewed in the image). The historic commission will not allow any part a new roof structure to be higher than the existing historic roof, but they will allow flat roofs in order to meet that regulation.