On November 6, 2015, about 1710 eastern standard time, a Cessna P210N, N731LT, was substantially damaged while landing at DeKalb-Peachtree Airport (PDK), Atlanta, Georgia. The private pilot and one passenger were not injured. The airplane was registered to Horst Aviation, LLC, but operated by an individual under the provisions of 14 Code of Federal Regulations Part 91 as a business flight. Visual meteorological conditions prevailed at the time of the accident, and the flight was operating on an instrument flight rules flight plan. The flight originated from Smoketown Airport (S37), Smoketown, Pennsylvania, about 1330, and was destined for PDK.The pilot stated that he expected and was set up for an RNAV approach, but was cleared for the ILS approach to runway 21L. He continued inbound to PDK and upon reaching the final approach fix, he recalled lowering the landing gear, but because of the approach distraction he did not verify that the landing gear was down and locked. He continued the approach, and reported breaking out of the clouds at 2,000 feet. When the airplane was at 200 feet above ground level, he reduced power and did not hear any warning horn. He indicated the landing was normal and very smooth, and after rolling about 400 feet, he heard a "snap" and felt the airplane drop and veer to the left. The airplane rolled off the runway onto grass and came to rest with the nose landing gear down and locked but both main landing gear collapsed. The airplane was raised, and both main landing gear were extended for towing to the ramp. Following recovery of the airplane, examination of both main landing gear actuators revealed no evidence of leaks or hydraulic residue. A test gauge was plumbed into the aircraft's landing gear hydraulic system and held pressure (1,500+ psi) in the up and down position for more than 10 minutes. The airplane was placed on jacks and several fault-free gear cycles were performed including an emergency extension. The single landing gear down and locked light functioned normally, and left main landing gear down limit switch which felt, "a little sticky", was replaced. There was no report of any damage to either main landing gear downlock hook assembly. A check of the landing gear warning horn revealed it was set 0.3 inch above the flight idle gate, while it is specified to be set 0.6 inch above the flight idle gate. It was adjusted to the specified amount, and although a flight check of the landing gear warning system was not performed during a postaccident maintenance flight check, the repair facility reported it was performed by the owner on the first flight after completion of repairs and no discrepancy was reported. The airplane's landing gear was hydraulically controlled, and by design the nose landing gear extended aft while the main landing gears extended forward. A representative of the airplane manufacturer reported that during landing gear extension, the nose landing gear locked into place followed by the main landing gear. This was because the nose landing gear extended aft and was assisted by airloads, while the main landing gear extend forward against airloads. During retraction of the main landing gear, the wheel assembly drops about 12 inches below the position when fully extended. Fully locking down of the main landing gear actuator occurs with a downlock hook assembly installed on each main landing gear. The airplane's last annual inspection was completed on February 26, 2015. There were no reported discrepancies during cycling of the gear that was performed during the inspection. The airplane had accrued about 66 hours since the inspection was performed.