The restoration work on Leinster House is going to cost more than the estimated €15m.
The cost overrun was outlined by Minister of State Kevin 'Boxer' Moran at the Oireachtas Finance Committee this afternoon.
Asked by Fianna Fáil’s Michael McGrath how much the structural work will ultimately cost, the Minister said he did not have a final figure.
The Chair of the Office of Public Works, Maurice Buckley, told the Committee that additional works had been anticipated due to the age of the Georgian building.
He maintained an accounting method had been put in place to limit any overruns and they were "very solid".
Mr Buckley said architects had gone through 90% of the building - suggesting the price would be agreed when that was completed.
Mr Buckley earlier told the Committee that there was "rampant construction inflation" in Ireland which was running higher than the 7.4% projected figure - probably closer to 12%.
Asked by Labour’s Joan Burton how he would deal with €3m in required savings in his Department, due to the cost overrun in the National Children’s Hospital, Minister Moran said he was "confident I will deliver" on flood defences.
He said some flood-relief projects may have to be delayed but that would only be for "a few weeks".
The Minister agreed to return to the Committee when he finalised his €3m in savings.