The blaze after 4 a.m. Wednesday started as a small fire behind a bedroom dresser. Residents tried to put it out with hoses and cups of water, but the fire quickly spread and eventually charred most of the house.
Firefighters on scene said an electrical problem likely started the blaze.
Seven people inside the house and five others who lived in a room behind the house escaped.
San Antonio Fire Battalion Chief Mark Black tells KSAT-TV that the home is a "complete loss."
Resident Graciela Galvan tells the San Antonio Express-News that the family is "only left with what we're wearing." She's not sure what the family will do next.