Hayes-White said 140 firefighters were spraying nearby buildings to prevent the blaze from burning other structures.
PHOTOS: San Francisco fire
"This is a great save right here," Hayes-White told reporters. "To be able to save this is huge."
She said a perimeter had been set around the fire scene to keep people away because "the possibility of collapse is very likely." The chief said sprinklers had not yet been installed in the burning building, which was to contain hundreds of condos that were to be put up for sale later this year.
"We are not quite contained, but we will be there a in few more hours," the fire chief said.
Residents in nearby buildings said the heat had broken windows, forced evacuations and could be felt more than a block away.
Some workers reported a scary, smoky commute home, and others took to Twitter to joke that the blaze might be an anti-gentrification plot.
The fire, reported just before 5 p.m., was burning in a neighborhood that includes office buildings, residential units and biotech firms.