I have this working in that I can get all the external lines using the offset method. I can then extend these (a nominal length) at either end so that they intersect. But the issue is then trying to fillet these together or to do a batch intersection. This would return a series of points which you could then use to create a new polyline. But Dynamo doesn’t seem to have a batch intersection function that return points (?). Might need to be done with Python recursively.
Another way to look at it is to get the solid geometry of the walls (and external doors), create a solid union, and intersect it with a plane. Filter the 2 resulting perimeter curves by area to get the exterior perimeter polycurve. But then you potentially have lots of little curves in the polyline as described here: