A teenager expelled from a hostel for damaging it by fire returned and set light to a foam mattress, York Youth Court heard.
The youth, who was 16 at the time, climbed through a window of the three-storey building, said prosecutor Mike Duffy.
He set fire to a foam mattress in his former room on the top floor where he had already caused fire damage to a windowsill and ceiling.
The youth pleaded guilty to two charges of arson committed in July in south York.
The Crown Prosecution Service dropped an allegation that he started a fire being reckless as to whether life would be endangered.
Youth justices warned him they had to consider a custodial sentence and remanded him in custody for three weeks. He has already spent six weeks in two separate periods on remand in a young offenders institution.
His solicitor, Philip Brown, said: "He is someone who had no experience whatsoever of the prison regime and has found that particularly difficult."
But he was unable to make a bail application for practical reasons.
He reserved the youth's "considerable" mitigation for when he appears for sentence.
Mr Duffy said the teenager started living at the Victorian three-storey house in January. It is home to several people.
In July, the landlord ordered him to leave because of his behaviour, which included making loud noises and damaging a windowsill and ceiling by fire.
When told to go the teenager left the building, but later returned via a ground floor window to his room.
He set fire to the mattress and left the shower running with the outlet blocked by paper.
Mr Duffy said the youth had already spent six weeks behind bars since he was arrested in the hostel in July.
At one point he was granted bail with conditions that included staying at a bail hostel.
But he breached his bail and was returned to custody.
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