The fire started shortly after 10 a.m. PT and gutted five units at the Highlands Retirement Residence. Eighteen more units have smoke damage.
No one was seriously hurt in the fire but a few residents were treated for smoke inhalation.
Deputy fire chief Thomas Doherty said the fire started accidentally when a cigarette was thrown into a planter outside the facility's back door.
"An accumulation of combustible material in the immediate area of the fire added to the fuel load which caused the fire to quickly spread throughout the structure," said Doherty in a news release.
He said it's still not clear when residents will be able to return home.