The home in the 9700 block of 1st Avenue NW caught fire at about 1:30 a.m. Monday.
Firefighters say a man, woman and 8-year-old child were already out of the home when fire crews arrived at the scene. All were taken to Harborview Medical Center as a precaution.
One firefighter also suffered minor injuries.
The home was heavily damaged by the flames. Investigators estimate the loss at $450,000.
An overheated extension cord under a deck was determined to be the cause of the blaze. Smoke detectors alerted the family and allowed them to make a safe escape.