The blaze broke out at a three-story townhome in the 4800 block of South Graham St. shortly before 1 p.m., and spread to the neighboring building.
Raging flames could be seen torching the roof of one structure, and heavy plumes of smoke were rising from the second adjacent building. A third building suffered heat and smoke damage.
"Once we learned a second home was exposed to the flames, we added additional engine companies and we wanted to make sure that we knock this down before it did spread any further," said Kyle Moore with the Seattle Fire Department.
No one was hurt.
Moore said investigators determined someone was baking cakes in a backyard shed using an open propane flame and left the area unattended. A fire started and then spread to the townhouse.
Vinh-Khanh Hoang says she was home on the second floor when she spotted smoke and then flames.
"We were just cooking and then I think a bag caught on fire," Hoang said.
The Hoangs said there were several propane tanks stored in the shed, which really fueled the flames.
Damage estimates were not yet available, but fire officials say the main unit is likely a total loss.