Large chunk of house doesn't load in multiplayer

I've been hosting a server via Steam for myself and a friend to play, and we've been doing so for a couple of months now. Today when I tried opening it I got a bit scared because a large chunk of the house we've made simply didn't load in. NPC:s are glitching up and down as they seem to almost register the floor of their separate rooms.
Thing is, this only appears to be a problem in Multiplayer, as the problem doesn't appear in Singleplayer. At first there was an instance where the world...

Large chunk of house doesn't load in multiplayer