Welsh assembly figures show that preliminary work was the most costly at £2.9m – the original cost was estimated at just £2m. But piling work cost nearly three times more than expected, at £495,000.
The assembly responded by bringing in QS Simmons Group to estimate costs for the remaining 29 packages. The QS calculated that these would exceed their budgets by between 30% and 100%.
RRP is understood to have sat in on the meetings where the figures were discussed, but did not attend subsequent discussions, during which its fate as lead partner was decided.
The architect has disputed the assembly's claims that the landmark building would have come in at least £14m over its £27m budget.
A spokesperson said it had been in negotiations with tenderers over the cost of future packages. He said: "RRP was in discussions with the tenderers to develop detailed solutions that fell in line with the pre-tender estimate."
It’s impossible to comment on the final cost of packages for which the contracts have not yet been awarded
Richard Rogers Partnership spokesperson
He added that a project could not be judged on a small number of packages. He said: "It's impossible to comment on the final cost of packages for which the contracts have not yet been awarded."
The Welsh assembly and RRP have declined to release the projected figures from the report.
But a Welsh assembly spokesperson did deny speculation this week that RRP may be reinstated on the project. He said: "The current position is that we've agreed to seek developers to come in and take over."
The spokesperson added that the assembly will be retendering the project by the end of September.
A dozen architects and developers are known to have already informed the assembly of their interest; these are believed to include RRP itself and Grosvenor Waterside, the assembly site's freeholder.