The National Emergencies Commission (NEC) say 12 other people are missing in the disaster in San Antonio de Escazu, a suburb seven kilometres south-west of San Jose.
It says a wave of mud and rocks rushed down the Pico Blanco hill near the suburb at dawn and slammed into several homes.
“They are now talking about 20 people dead,” a spokesman for the agency, Reinaldo Carballo, said.
Earlier, officials had put the death toll at 12.