Stuart Hazell has been jailed for a minimum 38 years for killing his partner's granddaughter, 12-year-old Tia Sharp.
The 37-year-old had denied murder, however on the fifth day of his trial at the Old Bailey he changed his plea.
The court heard Hazell murdered Tia last August after sexually assaulting her.
Judge Justice Nicol said he could not be sure the motive for the murder was sexual or that it was pre-meditated.
He hid her body in her grandmother's loft in New Addington, south London.
Despite four searches of the house, her body was not found until a week after she had been reported missing.
Hazell had claimed Tia died after falling down the stairs and breaking her neck, and he hid her body because he panicked.
The prosecution claimed he had a sexual attraction to the schoolgirl.
Jurors were shown an explicit image of a child, alleged to be Tia, which was said to have been taken in the early hours of 3 August 2012 when prosecutors say she died.
The court heard Hazell had previous convictions stretching back 22 years for offences including drug dealing, racially aggravated common assault, burglary and causing grievous bodily harm.
In a statement read out in court, Tia's mother Natalie Sharp said: "I gave the ultimate trust to Stuart.
"Sometimes I feel pity, that I want to hurt him, but I can never hurt him like he has hurt me."