Rogers sent a six-page letter to Welsh Assembly members last week, giving reasons why the Richard Rogers Partnership should not have been sacked.
In July, finance minister Edwina Hart said RRP had permitted costs to increase from £27m to about £40m. But the letter from RRP dismissed this claim, saying independent project reviews confirmed that the cost estimates were accurate: "The original cost plan was not underestimated," it said. "Three separate firms appointed by the assembly reviewed the scope, content and work contained in our cost plans. All three generally endorsed our costings."
A further cost review undertaken by Symonds earlier this year suggested that RRP had permitted the construction part of the budget to rise from £13.1m to £24m. But the letter claims that the Symonds review reached that conclusion because it included items beyond the scope of RRP's responsibilities.
The RRP letter states: "The comparison between the original cost plan of £13.1m and the £24m is illegitimate. The two numbers represent different scopes of work. The £24m contains a host of client items that must be added to our original cost plan for any true comparison to be made."
RRP claims that changes requested by the client, including the use of Welsh slate, had led it to warn of a £2.5m increase in the cost plan as early as last November.
The original cost plan was not underestimated
Richard Rogers Partnership
Hart issued a rejoinder to the latest RRP claims in a memorandum to assembly members. She said the architect's warning of an increase in its cost plan proved that it could not deliver the project on budget as it had previously claimed.
This spat reflects disagreement between the parties over RRP's responsibilities.
Last week, Building reported that the terms of RRP's contract did not seem to be clear.
RRP's letter said it was "prepared to enter into a dialogue to resolve these issues to enable the building to be completed to our original design".