At the conclusion of an uneventful multi-engine training flight, the pilot receiving instruction and flight instructor returned to the departure airport. They entered the traffic pattern with the intention of practicing a simulated single-engine inoperative approach to landing with the right engine simulated as inoperative. The pilot reported that after he extended the wing flaps on short final, he thought the airplane’s rate of descent was excessive. He applied corrective throttle inputs to the left engine, but at about 5 ft above the runway, decided to initiate a go-around by applying full power to the left engine only. The airplane turned aggressively to the right, and the flight instructor attempted to regain control of the airplane. However, the airplane continued to the right of the runway and impacted the adjacent grass covered surface before colliding with a hangar. The airplane sustained substantial damage to both wings. The flight instructor stated that although the airplane was slightly slow, he had determined that the landing was assured, so he was not expecting the pilot to perform a go-around. He briefed before landing that the pilot could use both engines during the landing rollout but did not explain that the pilot could use both engines for a go-around. The pilot and flight instructor reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.