Someone set the house on fire. The house was set on fire. Perhaps a little more informal.... Someone torched the house.

The last one most closely means that it was intentional. But Jon is right. By saying "someone lit the house on fire" or "someone burned the house" is a better way to say that. P.S. The word "arson" is the act of intentionally setting something on fire.

Can't you just say "someone"? None of those imply whether or not it was accidental or intentional. "Someone burned down the house" - This, at least implies, that is was intentional.

There was arson involved in the house fire