In a written statement, the pilot reported that shortly after departure, with the airplane about 30 to 40 feet above ground level (agl), the left door opened. The gull-wing type door prohibited the pilot from being able to reach its latch without him unfastening his seatbelt. Given the airplane was only about 3,000 feet down the 8,000-foot-long runway, the pilot opted to abort the takeoff by retarding the throttle to the idle position. The sun was reflecting off the windscreen hindering the pilot's vision and he could not ascertain the airplane's height above the runway surface. The airplane landed hard, incurring structural damage to the airframe. The pilot stated that accident could have been prevented if there was more emphasis in "door operation" when performing a check in a different make and model airplane. The pilot further added that there were no preimpact mechanical malfunctions or failures with the airframe or engine.