It might be useful to say that when your game has bugged out, like it did to me, you don't have to restart the game all over.
When Ratbeard doesn't give you the Snow building blueprint when you have finished the metal trader in the forest kingdom, you cannot finish your final structure, there is also now way you can get it on YOUR save.
So here is what you should do, like I did. You join a multiplayer game where the host is in chapter 8. You go see if he has the final structure. If he does you ask him politely to leave the game, so it makes you the host. This way you can demolish the chair, pick one piece up and build it again. Pling, achievement unlocked.