A two-story building in New Fairfield sustained serious structural damage in a fire yesterday afternoon.  Firefighters responded to a self-storage building on Dunham Drive shortly after 3pm.  The call was initially believed to be a car fire, but emergency personnel were told that the building was involved.  Firefighters held the flames in the original unit where the fire started.  No injuries were reported and the New Fairfield Fire Marshal's Office is investigating.  Mutual aid was provided by Danbury, Sherman, and Putnam Lake New York fire departments.  A Danbury Fire Department truck was used to gain access to the roof and overhaul the exterior of the building.  Tankers assisted with water supply.