Game : Carcassone with expansion "Abbey & Mayor".
Question - theoretically barn can be placed:
A) only if my farmer has already been placed 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.