Investigators believe the fire, which was reported in the 5600 block of Silverado Trail near Yountville, was electrical, Fire Marshal Pete Munoa said.
The 400-square-foot structure that burned had no internal electric service, and an electric cord from a garage 20 feet away was connected to a lamp inside the small outbuilding, Munoa said.
"It wasn't a permitted dwelling. It was built illegally," he said.
About 30 Himalayan and Persian cats, valued at $500 each, perished in the 4:20 p.m. fire, Munoa said.
The cattery was formerly known as South Paws and had been in business about 10 years, Munoa said.
He said a 1,600-square-foot residence on the property was not damaged.
The husband of the woman who operated the cattery was watching football with his son when he heard a popping sound, looked outside and saw the small structure on fire, Munoa said.
The man used a garden hose to prevent the fire from spreading while his son opened the door to the burning building, Munoa said. Two cats escaped and survived.
Another door in the structure was blocked by boxes, Munoa said.