Members of the Commons and Lords will be asked to choose between moving out of the Palace of Westminster entirely, at a cost of up to £15.6 billion over as long as 24 years, and staying partly in place under a plan called "enhanced maintenance and improvement plus", at a cost of up to £39 billion over as long as 61 years.
The two routes are the recommended survivors of a four-option list that both Houses are expected to narrow to two. They were set out in a report from the Public Accounts Committee published on Wednesday, in a Press Association report carried by The Irish News.
The committee said the weekly cost of maintaining the parliamentary estate is expected to grow from the current £1.5 million to £2 million in coming years, and that each year the programme is delayed will add £420 million to the lifetime cost of delivering it.
Members first voted to leave the building for extensive repairs in 2018. There has been continued disagreement since about whether the move should go ahead, and a series of decisive votes expected around Easter this year has been delayed.
Both Houses must also sign off on "Phase One" of the programme, capped at £3 billion, which would move the Commons and Lords to nearby temporary locations and relocate offices outside the palace. Under current timescales that departure would happen in 2032.
The committee warned that the temporary homes may not be ready when members need to leave, and asked the Restoration and Renewal authorities for a more detailed delivery schedule that takes account of the point at which the accommodation is actually needed. It pointed to Canada's experience of renovating its own parliament buildings as a guide.
It also said the programme needs to be more publicly accountable and transparent, recommended setting up a Restoration and Renewal select committee to scrutinise it, and said members need more engagement from the authorities so that they understand what their choices will mean.
Sir Geoffrey Clifton-Brown, who chairs the committee, said: "I would urge all those who believe a decision on how to proceed should be further delayed to digest the figures in our report, showing each kick of the can down the road costs hundreds of millions in public money."
He acknowledged that all the options "involve extremely high figures with very big ranges", and said that at a time of national fiscal constraint the parliamentary authorities would be expected to examine the figures extremely carefully to see how they could be reduced. A UK Parliament spokesperson said the committee "concludes that without action, the Palace of Westminster will, at best, be in a process of managed decline".
The figures reward a little arithmetic. At £2 million a week, 61 years of maintenance comes to about £6.3 billion. The option that lasts 61 years is costed at up to £39 billion. The option that lasts 24 years is costed at up to £15.6 billion.
The delay figure completes the picture. Each year of delay adds £420 million to the lifetime cost, so 37 years of delay would add £15.54 billion, which is the full move, delivered by not deciding on it.
This newspaper notes that the committee has therefore priced inaction at the figure of the works and given it a longer schedule. A programme nobody has chosen still has a delivery date, and on these numbers it is 2063.
The recommendation to establish a Restoration and Renewal select committee is the most promising element in the report. A select committee is a procedural step. Procedural steps take time. Time, in this programme, is costed at £420 million a year.
The Department of Unnecessary Research has written to the committee for clarification on the one point the report leaves open. If scrutiny of the delay forms part of the delay, the new committee's running costs must be set against the £420 million it has been created to scrutinise, and the Department would like to know which column they belong in.