The teens were experimenting with explosive materials - possibly fireworks - when the explosion occurred.
One adult was in a back room of the house at the time, and told police he wasn't sure what the teens were doing. The relationship of the man to the teens is unclear.
Both teens were taken to the hospital for treatment of lacerations to the arms, and one to the head. The adult was uninjured.
Seattle police's Arson & Bomb Unit is investigating.