'House of horrors' Builders find new secret dungeon in Fritzl guesthouse

Builders made the horrific discovery of a secret basement in Josef Fritzl's house almost a decade after he was jailed for keeping his daughter captive.

The case of the notorious Austrian rapist has made headlines around the world after he imprisoned his daughter Elisabeth in a cellar, sexually assaulting her over 3000 of times of a 24-year period and fathering seven children with her.

The discovery of the secret basement was made while workers were measuring the guesthouse during a routine building inspection.

Fritzl was sentenced to life imprisonment. Source: Getty Images

They found a staircase behind an improvised wall.

In addition, they found some glass tiles in front of another wall.

It's unclear whether police will investigate further or not, The Mirror reports.

According to local media the house was never properly inspected by the police even after they learned about Fritzl's crimes in his next property.

Fritzl ran the ‘Seestern’ guesthouse along with his wife Rosemarie in the town of Unterach am Mondsee before the couple moved to Amstetten in 1996.

PICTURED: Elisabeth Fritzl. She was just 18 years old when she disappeared. Source: Supplied

He kept his eldest daughter there as a sex slave in a cellar, which included a kitchen and bathroom.

His daughter had seven children without any medical help.

Three were sent "upstairs" and lived relatively normal lives.

It's believed his wife had no idea about the situation and was told by the serial rapist that their daughter had run away to join a religious sect.

She was told that Elisabeth had dumped her newly-born children on their doorstep.

One of the underground rooms found by police where Elisabeth was held. Source: Getty Images

One child died after Fritzi let it die from breathing problems and the other three were not allowed to leave the cellar.

When the eldest daughter fathered by Fritzi, Kerstin, slipped into a coma, he took her to hospital where doctors noted her malnourished condition and rotting teeth.

Elisabeth was granted permission to leave cellar and visit Kerstin in hospital but she was arrested and it was then that she told police about the shocking crimes her father committed.

The home where Fritzl held Elisabeth captive. Source: Supplied

Fritzi is currently being held in a special facility for ‘mentally abnormal criminals’ at Austria's Stein prison.

Elisabeth, 51, has a new identity and lives in ‘Village X’ near to the house where she was held along with her remaining children.