What do you advise me and my father regarding building a room within the fence of the house that was built with usury, knowing that my father took the loan before knowing of its prohibition?

If your father borrowed this money before knowing the prohibition of usurious loans, then there is no harm in living in and benefiting from the house, provided there is an intention not to return to usury and to repent for any negligence in seeking knowledge. The Permanent Committee has issued a fatwa stating that a house built with usurious money should not be demolished if the borrower was ignorant of its prohibition, with the obligation of repentance and seeking forgiveness.