AMSTETTEN - THE Austrian father who imprisoned and repeatedly raped his daughter over a 24-year period planned the improvised dungeon six years before taking her hostage, police said on Monday.

Josef Fritzl, 73, imprisoned 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 moustachioed septuagenarian, dubbed a 'monster' in the Austrian press, received planning permission to extend his house at 40 Ybbsstrasse in 1978, and worked on it for five years.

Fritzl has confessed to imprisoning and raping his daughter, now 42, and will be interviewed by investigating prosecutors later this week, possibly on Wednesday or Thursday, spokesman Gerhard Sedlacek said.

Fritzl's macabre double-life only became public knowledge on April 26th, when police discovered the highly protected basement to his ordinary terrace house in Amstetten, west of Vienna.

Fritzl had imprisoned his daughter Elisabeth, 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 told reporters on Monday.

Two of the doors were of reinforced steel, whilst an earlier entrance to the original cellar, not thought to have been used after Fritzl expanded the living space, was reinforced with concrete and weighed half a tonne, police said.

Fritzl is currently being held in St Poelten prison, where he is a 'unproblematic inmate ... calm, collected and alert,' according to prison director Guenther Moerwald, cited by APA newsagency.

His lawyer has said he would be recommending his client pleaded insanity.

Although Fritzl is being separated from other prisoners for fear of violence, he does share his cell with one other inmate, who agreed to the arrangement, Mr Moerwald said.

Fritzl is taking meals in his cell, and refusing to leave for the voluntary one hour's yard exercise permitted each day.

His complex familial set-up saw him father seven children with his wife Rosemarie, 69, whilst fathering seven more - one of whom died shortly after birth - with daughter Elisabeth, locked in the underground cellar.

Three of those children were then fostered by the upstairs family. Fritzl told police Elisabeth had joined a religious cult and had deposited the babies with their 'grandparents' for bringing up.

The other three grew up entirely in the enclosed, windowless cellar.

Psychiatrists and carers said on Monday 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. Their original cuddly toys have also been given to them.

Up to 30 police officers have been investigating the crime scene.

Mr Polzer speculated that Fritzl's motive for the decades of abuse was to replicate his above-ground family with a younger, prettier woman.

'His motive was to recreate once again the situation he had with his first family, the legal family, but this time with a good-looking young daughter,' Mr Polzer said on the sidelines of the press conference.

In each set of children, there was even a pair of twins, Mr Polzer said.

'The daughter had twins at the same point as there had been twins in her own family, only one generation earlier. It's a remarkable parallel,' Mr Polzer said.

'As a crime scene investigator, I can say this is a unique case,' he added. 'This was an exceptionally active lone criminal.' -- AFP