Forensic teams said they discovered a series of underground rooms, equipped for sleeping and cooking, and with sanitary facilities.
Franz Polzer, a criminal investigations officer, said the door was "very small", and that one had to bend one's head to get through.
"Everything is very, very narrow and the victim herself, the mother of these six or seven children, told us that this was being continually enlarged over the years," he said.
The 42-year-old woman, identified as Elisabeth F., had been reported missing since August 29, 1984.
She was found by police in Amstetten on Saturday evening after police received a tip.
Elizabeth F told police that she had been abused since the age of 11, seven years before being imprisoned.
She accused her father of "massive crimes."
Fritzl allegedly lured his daughter into the basement of their home in the town of Amstetten, anaesthetised and handcuffed her and then kept her captive.
Elizabeth F said that during the 24 years that followed, she gave birth to six children.
In 1996, she gave birth to twins, but one died several days later because it was not properly cared for, according to police.
Fritzl then apparently removed the corpse from the cellar and burned it on the incinerator of his apartment building.
Rosemarie, Fritzl's wife, was apparently not aware of the events.
Austrians have reacted with shock and outrage.
Guenter Pramreiter, who owns a bakery on the same street, said Fritzl and his wife would regularly buy bread and rolls, though never in large quantities.
"They appeared normal, just like any other family,'' Pramreiter said.
"I'm totally shocked, this was next door. It's terrible.''
Residents of the neighborhood, which is a middle-class district, said nothing in their contacts with the family suggested anything unusual.
The suspect was described as friendly be neighbours: "That's why this is so unbelievable," said Franz Redl, 56, who owns a shop across the street.
"I'm sure the authorities did all they could. He planned everything so perfectly.''
Fritzl is expected to appear in court later on Monday.