Firefighters and police responded to the scene, near NE 144th Street and 232rd Avenue NE, at about 3 p.m. after receiving a report of a plane crash into a single-family home.
Two people were aboard the plane. The pilot, Jay Uusitalo, 45, of Redmond, was found dead at the scene.
The passenger, a teen from Eastern Washington who is Uusitalo's nephew, was rushed to Harborview Medical Center with severe lacerations to the head and chest, as well as internal injuries. He initially was listed in critical condition, but was upgraded to serious on Saturday.
Two teenage boys were inside the home at the time, only feet away from the spot where the plane crashed.
In an interview, they said they heard a loud crash and the whole house shook. They then saw the wing of a plane inside the home.
"I was sitting in my house, and I hear a loud bang. And I - like thunder - I was thinking thunder," said one of the teens. "I look outside, and I look down, and there's the wing right down below it, below the window. And I was right there, too. I didn't realize how close it was to hitting me."
Other witnesses who live in the neighborhood say they heard sputtering and looked up. They saw the propeller on the plane stop and the aircraft flipped upside down, crashing into the home.
According to the Federal Aviation Administration registry, the fixed-wing single engine plane is registered in Redmond and it seats up to four people.
It was not immediately known where the plane was coming from or where it was headed.
The FAA, National Transportation Safety Board and the King County Sheriff's Office are all investigating the crash.
The plane's wreckage likely will not be removed until Sunday, officials said.