The report was commissioned by the country's environment ministry as a balance to a report earlier this year which concluded that it would be more economic to demolish the tower, known as H-Block, and its best door Y-Block, both of which feature murals designed by Spanish artist Picasso, than to repair them.
"I strongly recommend preserving the buildings and give them a new use," Jørn Holme, the directorate's head, said in a statement. "There are no other buildings that can replace them. The H-block is of the best examples of monumental modernism in Norwegian architectural history."
The directorate's report questions the economics of demolishing and replacing the buildings, particularly if the five Picasso murals "The Beach", "The Seagull", "Satyr and Faun" and two versions of "The Fishermen" — are to be successfully preserved.
It also argues that if upgraded, the old buildings could be as environmentally efficient as a new building.
Anders Breivik launched his attack on Oslo on 22 July 2011 by detonating a car-bomb underneath the H-Block, causing the building severe structural damage, and shattering windows of buildings throughout the surrounding government compound.
Norway's government has yet to decide whether to follow Holme's recommendations.