The pilot reported that, while preparing to start the engine, he felt stressed and rushed because he was departing later than planned, and the weather forecast along his intended route was forecast to deteriorate as the day progressed. He followed the checklist, though after the accident he was unsure if he had properly set the parking brake or he may have inadvertently disengaged the brake with his leg/knee, which had occurred previously. After the engine did not start on the first attempt, he followed the flooded engine procedure, which included setting the throttle to the ½ position. When the engine started on the second attempt, the airplane began rolling forward. He attempted to stop using the brakes, but his sandals, which were wet, slipped off the rudder pedals. He further stated that “in a panic, I rapidly advanced the throttle instead of retarding it.” The airplane struck a building and sustained substantial damage to the right wing and engine mount. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.