DURING RETURN TO ARPT FROM A LOCAL FLIGHT, THE PLT DIVERTED TO OBSERVE PROPERTY OF A PERSONAL FRIEND, LOCATED ALONG A RIVER BANK. THE PLT WAS MAKING A RIGHT TURN ABOUT 200 FT AGL AT 40 TO 50 KTS AIRSPEED WHEN VIBRATION WAS FELT IN THE ANTI-TORQUE PEDALS & THE ACFT ENTERED AN UPRIGHT, VERTICAL DESCENT, SPINNING ABOUT THE VERTICAL AXIS UNTIL IMPACTING THROUGH THE ROOF OF A METAL STORAGE BUILDING LOCATED ON THE PROPERTY. AN INVESTIGATION REVEALED NO MECHANICAL FAILURE OR MALFUNCTION EXCEPT FOR A SLIGHT FLUCTATION IN THE FREE TURBINE GOVERNOR & FUEL CONTROL WHEN THE ENGINE WAS TEST RUN IN IT'S POST-CRASH CONDITION. THE PLT OF THE MEDICAL RESCUE HELICOPTER RESPONDING TO THE SCENE STATED HE HAD DIFFICULTY CONTROLLING HIS ACFT IN THE VICINITY OF THE CRASH DUE TO TURBULENCE AND DOWNDRAFTS IN A 20 KT WIND ASSOCIATED WITH DENSE FOREST ALONG THE RIVER.