On July 18, 2021, about 0850 mountain daylight time, a Beech 35-C33 airplane, N18AR, was substantially damaged when it was involved in an accident near Vance Brand Airport (LMO), Longmont, Colorado. The pilot and passenger sustained serious and minor injuries, respectively. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that the preflight inspection and run-up were normal and that, per the high-density altitude procedures, he leaned the fuel mixture before takeoff from the 4,799-ft-long runway. He rotated at 85 mph and initiated a climb; however, the airspeed and climb rate “did not look normal.” He was not certain that the airplane would clear a house located along the departure path and decided to execute a precautionary landing. He retracted the landing gear and entered a right turn to avoid the house. The airplane touched down and slid to a stop on its belly. The airplane impacted a hayfield and came to rest about 1/3 mile from the departure end of the runway. The fuselage and both wings sustained substantial damage consistent with the impact sequence. Post-recovery airframe and engine examinations did not reveal any anomalies consistent with a loss of engine power. Based on the 0855 weather observation at LMO, the calculated density altitude for the takeoff was 6,804 ft. Airplane performance data indicated that the expected takeoff distance to clear a 50-ft obstacle and the expected initial rate of climb were about 2,900 ft and 780 ft per minute, respectively. The pilot reported previous flight experience at high-density altitude airports that included flight instruction for such operations.