Before departing on the accident flight, the pilot called his friend and cancelled a previously scheduled flight for his friend's son stating that it was too foggy. About 30 minutes later, the pilot departed the mountainous area helipad on the accident flight. There were no eyewitness. The caretaker of the helipad area reported that the helicopter flew in a northerly direction and that the prevailing fog/low clouds conditions were intermittent at the time of departure. About 5 minutes later, ground witnesses heard the helicopter pass over the condominium complex in a southeasterly direction and moments later they reported hearing a sudden loud noise. The witnesses reported that the ceiling was about 50 feet above the ground and that the higher elevation tree tops were obscured by the clouds. The helicopter struck the trees south of the condominium complex and continued in a southeasterly direction until striking the southside of the complex. A 6-ft by 2-ft diameter section of the tree was severed by the helicopter's main rotor blade and penetrated the condominium complex roof and front glass window. The tree section came to rest about 250 feet beyond the tree impact area in the street.