Fire destroyed a small cottage early Thursday on Rose Avenue in Penngrove.

Firefighters found the one-bedroom structure fully in flames shortly before 2 a.m., said Rancho Adobe Fire Capt. Larry Martinez said. They contained the blaze to the residence and prevented its spread to nearby vegetation and structures in the neighborhood off Minnesota Avenue south of Old Redwood Highway.

When Rancho Adobe asked for extra water tenders, “a lot of help showed up,” Martinez said. Aid came from such fire departments as Petaluma, Rohnert Park, Wilmar, Gold Ridge, Bennett Valley, Schell Vista and Cal Fire.

Cause of the fire remains under investigation. It was uncertain whether the cottage had been occupied. The dollar damage has yet to be determined.