On September 8, 1997, at 1006 hours Pacific daylight time, a Bell 206B, N770GG, attempted to taxi to takeoff with the tie down chains still attached at the Napa, California, airport. The helicopter rolled over and one of the main rotor blades separated; a 3-foot section of the blade penetrated the side of a building and landed on a desk. The aircraft sustained substantial damage and the private pilot, the sole occupant, received minor injuries. Visual meteorological conditions prevailed at the accident site, and no flight plan was filed. The personal flight was originating at the time. Federal Aviation Administation inspectors from the Sacramento Flight Standards District Office responded to the airport, interviewed the pilot, and inspected the aircraft. The pilot reported to the inspectors that he did not routinely tie the aircraft down at night. Ramp personnel tied the aircraft down the night before due to high winds at the airport. The pilot reported to the inspectors that he performed his preflight and did not look down at the skids, nor did he notice the tie down chains.