The Tudor Hall at Jersey's Gorey Castle will be built with a roof after planners approved the latest development proposals.
There had been concern about the effect of roofing the hall
The Heritage Trust has welcomed the decision as a major milestone in the conservation strategy for the castle.
Plans for a roof and floors were approved in principle last year.
The planning authorities have now given the go-ahead to the plans which the Heritage Trust says will protect the fabric of the building.
Because the walls have been roofless, and therefore wet for many years, there had been concern that if the walls are roofed the stone could become too dry and draw up water from the ground.
The hall will now become a room with a wooden floor and ceiling and four contemporary windows and there will be exposed granite on the walls.