The Austrian man who imprisoned and repeatedly raped his daughter over a 24-year period planned the improvised dungeon five years before taking her hostage.
Josef Fritzl imprisoned his daughter Elisabeth in the cellar in 1984, when she was aged 18, but police revealed that he had started planning the dungeon as early as 1978.
The 73-year-old received planning permission to extend his house in 1978, and worked on it for five years.
Josef Fritzl has confessed to imprisoning and raping his daughter, now aged 42, and will be interviewed by investigating prosecutors later this week.
His macabre double-life only became public knowledge on 26 April when police discovered the highly protected basement to his ordinary terrace house in Amstetten, southwest of Vienna.
He had imprisoned his daughter, and then three of the seven children he fathered with her, behind eight locked doors.
The final three doors were opened electronically by secret code, police said.
Two of the doors were of reinforced steel, whilst an earlier entrance to the original cellar, not thought to have been used after Josef Fritzl expanded the living space, was reinforced with concrete and weighed half a tonne.
His lawyer has said he would be recommending his client pleaded insanity.
His complex life saw him father seven children with his wife Rosemarie, whilst fathering seven more - one of whom died shortly after birth - with his eldest daughter, Elisabeth, locked in the underground cellar.
Three of those children were then fostered by the upstairs family.
Josef Fritzl told police Elisabeth had joined a religious cult and had deposited the babies with their 'grandparents' to raise them.
The other three grew up entirely in the enclosed, windowless cellar.
Psychiatrists and carers said that the health of two of the underground children was gradually improving.
The eldest child however, 19-year old Kerstin, remained in an artificially-induced coma.
Her deteriorating health was what led Josef Fritzl to take her to hospital, sparking the doctors' fears which eventually led to his arrest.
The two other children, being cared for in a clinic, have been given an aquarium, to replicate the one Fritzl allowed in the cellar.
Up to 30 police officers have been investigating the crime scene. The police chief speculated that the motive for the decades of abuse was to replicate the above-ground family with a younger, prettier woman.
