The body that runs the Welsh Assembly is considering replacing the windows at its rented offices at a cost of £4m.
The Assembly Commission said the windows in Tŷ Hywel, next to the Senedd in Cardiff Bay, have "an increasing level of faults and failures".
The building's "coastal environment" has led to "a reduced operational life" for the 28-year-old windows, it added.
Tŷ Hywel is rented while the Senedd building, containing the debating chamber, is owned by the assembly.
The Tŷ Hywel building, which houses offices and committee rooms, is owned by London-based Equitix Tiger English LP, a limited partnership registered with Companies House in December 2018.
According to the commission's budget report, the windows of the type in Tŷ Hywel have an expected operational life of 25 to 35 years but "the building's location in a coastal environment has led to a reduced operational life".
Independent surveys have identified that the windows are in "poor physical condition and that thermal performance is likewise poor", the report said.
"An increasing number of windows have faults and replacement parts can no longer be obtained to carry out repairs."
Explaining why the commission as tenants, rather than the landlord, is responsible for the expenditure, the commission said that "as part of a full repairing and insuring lease agreement for Tŷ Hywel, the commission is required to maintain the building in a good state of repair, including replacing any end of life items such as boilers, doors and windows".
During a debate on the commission's budget in the Senedd last month, the commissioner responsible for budget and governance, Conservative AM Suzy Davies, acknowledged "the windows project... has come as something of a surprise, perhaps, to some assembly members, but it was never going to be cheap".
A spokeswoman for the Assembly Commission said: "The decision regarding the work will be taken by the new commission in the 6th Senedd [which begins after the 2021 election] and this will include a decision around expenditure and phasing of the project in order to get the best value for money and address sustainability concerns."
What is Tŷ Hywel?
- It was originally named Crickhowell House after the former Conservative Welsh Secretary Lord Crickhowell but has since been renamed after medieval Welsh ruler Hywel Dda
- It was built in the 1990s by Grosvenor Waterside, the property arm of Associated British Ports, which owned large areas of land around the newly reclaimed Cardiff Bay
- The Welsh Office took out a lease on the building and it was transferred to the assembly when it was established in 1999
- It housed the assembly's Siambr - debating chamber - until the Senedd was completed in 2006
- The commission says it "remains a long-term objective of the commission to secure ownership of Tŷ Hywel at some stage in the future"