London, May 6 : Josef Fritzl, who imprisoned and repeatedly raped his daughter over a 24-year period, planned the improvised dungeon in 1978 - when she was only 12 years old.
According to police chief Col Franz Polzer, the 73-year-old electrical engineer had the intention to build a small space unknown to the building authorities.
"He had the intention to build a small space, a small secret, a small dungeon unknown to the building authorities," Telegraph quoted ol Polzer, as saying.
"The logic says the idea was already there, or an obsessive thought played a role, to build this jail, dig out these rooms and later to equip them and imprison the daughter there," he added.
Fritzl protected the hidden space with eight doors, secured with sophisticated electronic locks.
These included a separate main door that weighed half a tonne and is thought to have been unused for years because of its weight.
Col Polzer said: "The door was hidden in a room beneath the stairs. We had to call in the fire brigade to help us open it."
Elisabeth, 42, spent 24 years in the dungeon and three of her children - Kerstin, 19, Stefan, 18 and Felix, 5 - have lived underground their whole lives.