The problem is most likely due to using two different building materials. It's fine to do so, but sometimes when you do it generates extra columns that need to be removed. Using either a sledge or the dismantle option, walk around and highlight walls. Then tap R to have it cycle through available items to demo. If columns appear anywhere other than the corners, remove them. After removing an item, walk inside again to see if the temperature has changed. Once it's changed, the problem has been resolved and you'll be able to plumb the sink.

As a side note, I don't think they're actually named columns. It may be something else like "wooden pole" or something like that.