The fire was reported shortly before 3:30 a.m. in the area of Pleasants Valley and Gates Canyon roads, Cal Fire spokesman Michael Selmi said.
Firefighters had the flames contained by about 6:45 a.m.
No injuries were reported, Selmi said.
Selmi did not know what type of structure was destroyed but said the small building was "well-involved" when firefighters arrived.
The cause of the fire remains under investigation.