The news came as QS Davis Langdon & Everest told the Fraser inquiry that it had privately informed the parliament's managers that the projected out-turn cost was £411m in August and £420m last week.
Building revealed at the end of last year that 264 design variations had been made on the scheme in November. Sources close to the project team said that further design changes had been made since Christmas – one source said these ran into the hundreds.
The source said: "There are still architects' instructions. Some are clarifications. But the team is still confident of completion by August."
The source also revealed the present state of the scheme:
- The MSP building and Queensberry House are "to all intents and purposes" complete;
- The administration and catering building will be finished at the end of March;
- The four tower blocks will be completed by the end of May;
- The debating chamber will be finished by the end of July.
The source added that the team's only concern was external work. He said: "They are worried about getting the ground green."
In his report on Tuesday, Reid said: "I deeply regret this further increase in costs to the public purse. Over the past month there has been major work completed on site, without which the cost rise would have been significantly higher."
One insider emphasised that safeguarding the building against terrorist attack had had a huge impact. He said: "It's had an enormous impact on the project, having reinforced glazing. You cannot blame that on architecture."