The 29-year-old will continue to serve the detention under house arrest with an ankle tag, the news agency NTB reported.
Hoiby was sentenced to four years in prison in June, though the verdict has not yet become final.
He was found guilty of two counts of rape and violence against a former girlfriend.
A restraining order is in place to protect her, which Hoiby has already violated on several occasions.
Police argued there was a risk of Hoiby committing further offences and requested an extension of the house arrest. The court agreed.
Hoiby sought release on condition that he stay away from certain areas, but the court rejected his application.
Appearing in court, he said: "I have done everything I can to show the prosecution that there is no risk of recurrence."
Hoiby intends to appeal the decision, NTB cited his defence lawyer, Petar Sekulic, as saying.
The court's ruling remains in force pending a decision on the appeal. The extended pre-trial detention runs until September 7.
Hoiby is permitted to leave his residence for purposes such as medical appointments or visiting sick family members. He must also submit to regular drug tests.
Hoiby is serving his sentence at Skaugum, the royal residence of his stepfather Crown Prince Haakon and his mother.
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