Complex sentence: Included is The Avery Coonley House which is the largest, privately owned Frank Lloyd Wright house to be recognized by the United States Secretary of the Interior and was declared a National Historic Landmark in 1970.
Split into simpler sentences: Included is The Avery Coonley House which is the largest, privately owned Frank Lloyd Wright house to be recognized by the United States Secretary of the Interior. It was declared a National Historic Landmark in 1970.