The fire was reported at 6:09 p.m. at a home in the 500 block of Willow Court, Novato Fire Protection District Deputy Chief Eric Nickel said.
Firefighters arrived minutes later and found the garage and part of the home in flames, Nickel said.
The blaze spread to the attic, and the roof partially collapsed while firefighters were cutting a hole in the roof to let some of the heat and flames out of the home, he said.
Nickel said firefighters had some trouble with the fire because a new metal roof had recently been laid over the house’s previous roof, so “it was very difficult for our crews to ventilate the home.”
There was also live hunting ammunition in the garage, but it did not explode, he said.
The fire was controlled at 6:55 p.m., according to Nickel.
No injuries were reported to firefighters or the occupants of the home, a husband and wife, he said.
The flames and heat have made the house “uninhabitable,” Nickel said.
A total of 44 firefighters from Novato and several other Marin County agencies responded to the fire, he said.
The blaze started in the garage, and its cause has yet to be determined.