The Scottish parliament has dismissed documents presented to the Fraser Inquiry last week showing another £20 million cost hike for Enric Miralles' Holyrood project.
Civil servants have rebuffed an official spreadsheet - by quantity surveyor Davis Langdon & Everest (DLE) - showing the latest cost estimate jumping to £421 million.
They are sticking to the latest official figure of £401 million, insisting that any further overruns will be offset against savings from a cap on consultants' and architects' fees and that VAT will be reclaimed.
The extra £20 million came to light after DLE partner Hugh Fisher presented the spreadsheet while giving evidence to Lord Fraser's inquiry. The final entry - of 8 August last year - shows a price tag of £411.39 million plus £9.5 million in extra payments.
But a spokesman for the parliament said there was no chance the official cost would rise again: 'DLE's calculations overestimate both VAT, £12 million of which we will reclaim automatically, and fees.
'The parliament's August total includes exact figures for these two elements as part of the £401 million estimate for the cost, ' he said.
However, Scottish National Party leader John Sweeney said he thought it likely the project would rise further.
'The public are sick and tired of having to fork out more and more cash to pay for this building and we must make sure that they don't have to, ' he said.