Crews responded to the scene, in the 11700 block of 15th Avenue NE, just before 11 a.m. after receiving 911 calls about the blaze.
All residents of the complex were evacuated. There were no reports of injuries, but six units were damaged and at least five residents were displaced.
The blaze reportedly started as a motorcycle fire that spread to nearby cars in the building's parking area and from there to several units on one side of the complex, said Kyle Moore of the Seattle Fire Department.
Aerial footage showed flames and smoke pouring from the upper story on one side of the structure before firefighters brought the blaze under control.
"I just saw a bunch of smoke and flames and fire trucks everywhere and people going crazy. It was pretty nuts," said witness Kyle McNary. "You could smell it, definitely. ... It's pretty shocking."
One resident, Leanne White, said she was at work when she heard about the blaze. She immediately thought of her dog trapped inside the building.
"I rushed over here, and I've been hysterical ever since I got her back," she said. "I saw pictures online and I freaked out. I was like, 'There's no way she survived.'"
But as it turned out, her dog, Fiona, did survive the fire - even though her unit is on the side of the building that most heavily damaged.
"I don't know that I have a home, but I've got my dog. Thank God," she said. "I was shaking the whole way, but I have what matters. I have my dog back, and nothing else matters. Everything else can be replaced."