Probably I misunderstood again.
For this purpose I've made an option in AutoDimensioning plugin, roof projection.
To modify this in 3D not only in 2D I try to do in the future for this plugin.
In AutoDimensioning wasn't simple to identify the exterior walls and set the build area or roof projection.
More than a half of the code is dedicated to this features.
The GenerateRoof it is already huge and I don't think it is useful to make it heavier adding something which already exist and work.
My yellow room is generated in this way. Then you could make fine adjustments even to add/remove points where you need.