In Remix 10, if you set Place in Kingdom to On, the buildings and decorations you earn in this mode will be automatically placed in your kingdom when you receive them. Items will be placed randomly in any empty spots. However, if there are no empty spots, the new items will take the place of previously placed items.
But don't worry, your items aren't gone! If something has been put away, you can find it by going to the main screen and tapping Build, and then Place.