Around £4.5 million is to be spent putting a new roof on Stormont's Parliament Buildings.
The entire roof of the 92-year-old building will be replaced and solar panels and rainwater harvesting will be installed, while accommodation on the fourth floor will be optimised.
The work will take at least 12 months and is expected to start at the end of the year. It will cause widespread disruption and means a number of offices will have to be temporarily relocated.
Pat Ramsey, a member of the Assembly Commission which has responsibility for property, staff and services at Stormont, said the refurbishment is essential because Stormont is a listed building.
"For a number of years there have been problems with water ingress to the roof in Parliament Buildings, causing damage to the fabric of this building which is a listed building. Short-term repairs have been made in recent years but it is now essential to undertake a major project to safeguard the long-term future of this building," he said.
The estimated cost does not include external fees.
The Assembly Commission has appointed a design team, including an architect, to develop plans for the new roof.
DUP MLA Ian McCrea said his office, at the front of the buildings, has mould growing on the walls on a "daily basis".