The one-alarm fire was reported at 8:09 p.m. at the Youth Guidance Center at 375 Woodside Ave. near Twin Peaks, which also contains courtrooms.
The blaze caused about $100,000 in damage to the building and $10,000 to its contents, fire spokeswoman Mindy Talmadge said.
Talmadge said the cause of the fire remains under investigation.
No one was in the building when the blaze started, and the area where it ignited was locked up, she said.
No firefighters were injured, and Talmadge said the juveniles in custody were in a separate building and were not affected by the fire.
Tony Gavero, court supervisor at the Youth Guidance Center, said court services are not affected today.