I prefer one big exterior room in those cases.
- Get the extents of a new 3D view and convert to a new bounding box.
- Move the maximum point by a reasonable distance on the + X and + Y axis, then pull it onto the plane you want to get the footprint at.
- Move the minimum point by a reasonable distance on the -X and -Y axis, then pull it onto the plane you want to get the footprint at.
- Draw a rectangle aligned to the XY axis using the relocated min and max points.
- Convert the rectangles to room separation lines. * Translate the min point by 1/2 of a reasonable distance on the + X and + Y axis, and create a room at this point.
- Name the room ‘outdoors’ and give it a number as desired.
- Get the boundary curve loops of the new room, and find the one with less area. That is your footprint.