PILOT STATED LEFT ENGINE LOST POWER FOLLOWED BY THE RIGHT ENGINE. HE INITIATED A LEFT TURN BACK TO THE AIRPORT BUT DID NOT SHUTDOWN ENGINES IN ACCORDANCE WITH EMERGENCY PROCEDURES. THE ACFT YAWED LEFT AND FELL OFF ON THE LEFT WING AS IF IT HAD STALLED. HE REGAINED PARTIAL CONTROL AND AIMED FOR A STREET. ACFT HIT ROOF OF HOUSE WHILE IN RIGHT BANK AND THEN CARTWHEELED ACROSS THE STREET. LEFT ENGINE WAS FOUND TO HAVE SEVERE DETONATION DAMAGE AND THE TURBOCHARGER WASTE GATE DOOR WAS MISSING ANTIVIBRATION SPRING. THE PIN CONNECTING THE DOOR AND ACTUATOR WAS FAILED. EVIDENCE INDICATED PIN HAD FAILED PRIOR TO ACCIDENT FLIGHT. SOOT INSIDE LEFT WASTEGATE INDICATED DOOR HAD BEEN FULLY CLOSED FOR SOMETIME. NO EVIDENCE TO INDICATE FAILURE OR MALFUNCTION OF THE RIGHT ENGINE WAS FOUND AND RIGHT PROPELLER SHOWED DAMAGE INDICATIVE OF ROTATION UNDER POWER AT GROUND IMPACT.