A report into the cost of the National Children's Hospital says most of the €450 million increase was due to an underestimation of the real cost of the project.
The PWCs report identifies a series of weaknesses in terms of set-up, planning, budget, execution and governance.
The report says "significant failures occurred during the crucial planning and budgeting stages of the project."
It says a situation was created whereby the approved project could "never be delivered within the financial parameters agreed."
The report recommends strengthening the rules that govern public sector spending and calls for an ongoing review of major projects.
It says re-tendering is not a feasible option and suggests there is little scope for savings.
The Taoiseach Leo Varadkar says the report makes for grim reading and the government accepts its recommendations.