The pilot attempted to start the single-engine airplane via the electric starter, but was unsuccessful. So the pilot elected to rotate the propeller by hand, to "loosen up" the engine enough to start. When the pilot rotated the propeller, the engine inadvertently started and the unoccupied airplane accelerated forward, until it impacted an open T-hangar. The accident airplane received substantial damage, the T-hangar was damaged, and a parked airplane received minor damage. The pilot added that the airplane's master switch and magnetos were not turned off, nor was the parking brake set prior to rotating the engine by hand.