@ winfixit, The building plugin can indeed be a hack, but still complicated manipulations for a presumably simple task.
I also found More tools > Align way nodes (Shift + L) - which comes with utilsplugin2, I think - does more or less what I have in mind, but again with an open line not to add a line that comes in continuation to a wall of an existing building. @ Martin, Indeed, we're talking about details. I'm asking if a feature exist and if not I'm suggesting it would nicely complement the many existing features in JOSM to draw geometric shapes (line, rectangle, circle, arc, etc.).
