Builders have discovered a secret basement in Josef Fritzl’s guesthouse almost a decade after he was jailed for keeping his daughter captive.

The notorious Austrian rapist imprisoned Elisabeth in a cellar, sexually assaulting her thousands of times over a 24 year period and fathering seven children with her. The case made headlines around the world.

But it now appears that he had built another concealed basement at his guesthouse just outside the city of Salzburg.

Workers found a staircase behind an improvised wall while measuring the building, according to local media. It was part of a routine building inspection.

It is unclear whether police will investigate the building.

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.

There, he would keep his eldest daughter as a sex slave in a cellar, which included a kitchen and bathroom.

She was also forced to bear his children without any medical help.

Three were sent "upstairs" as small children and lived relatively normal lives looked after by Fritzl’s unsuspecting wife.

The serial rapist told Rosemarie that Elisabeth had run away to join a religious sect and had dumped her newly born children on her parent’s doorstep. This was the same story he told to social services.

Whatever became of Elisabeth Fritzl? Show all 3 1 /3 Whatever became of Elisabeth Fritzl? Whatever became of Elisabeth Fritzl? 338454.bin Aged 18 when her father imprisoned her, Elisabeth Fritzl had seven children by him Whatever became of Elisabeth Fritzl? 27204.bin View of the back of the house where Josef Fritzl kept his daughter in a basement dungeon 2008 AFP Whatever became of Elisabeth Fritzl? 151483.bin Footage from 2009 shows Josef Fritzl being lead away after being sentenced to life in a psychiatric hospital AP

The other four were not allowed to leave the cellar which resulted in the death of one baby boy when Fritzl let the child die from breathing problems rather than call a doctor.

He then cremated his infant son in a wood-burning stove.

That property converted into a block of flats and the basement filled with cement by Austrian authorities, after Fritzl was sentenced to life imprisonment for the crimes against his daughter.

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