Carcassone - placing barn figure in field
According to the rules, the barn can be placed in a field as long as that field doesn't already have a barn. Farmers are no concern when placing the barn:


  You may place your barn on a field already occupied by farmers. You may not,
  however, place your barn on a field already occupied by another barn. Once
  placed, a barn remains in play until the end of the game.