I build walls using sectioned walls I can daisy chain together, and for stepping up cliffs I have to build a set of stairs between the two. I placed a section at the bottom and top of the cliff so I could line the stairs up correctly and when building the stairs on one, i accidentally connected it to the upper section which joined the two buildings together as one building. I hit the “undo” button, as I was not ready for the hearthlings to build both sections at once, and this resulted in a weird deformed section on the upper cliff that had half it’s pieces missing. I was unable to delete or remove the building and if I tried to “build it” it just generated an odd error message.
Welcome to the forum, @Atrum
Yep, this is an issue when joining buildings.
You seem to have a mod conflict, make sure you don’t have both a plain folder and a .smod file for the same mod, it could make the game more buggy after an update (plain folders take precedence over smods).