THE projected cost of the botched Renewable Heat Incentive scheme rose significantly after it was given the go-ahead.
An inquiry into the scheme heard yesterday that it was given the green light in June 2011 based on the findings of a draft economic appraisal.
However, by the time consultants submitted their final report several weeks later, the projected bill had gone up by more than £100m.
Official Peter Hutchinson told the inquiry it would have been "prudent" to advise then Department of Enterprise, Trade and Investment minister Arlene Foster that the estimated cost had risen because she had made her decision based on the draft report.
Senior civil servant Fiona Hepper has already told the inquiry she did not think it necessary to go back to Mrs Foster with the new numbers.
Mr Hutchinson said there may have been an expectation that all of this would be "teased out" at other points in the process.
The inquiry's chairman Sir Patrick Coghlin pointed out that no-one had raised the costing issue at a later stage.
The inquiry into the disastrous green energy scheme, which led to the collapse of the assembly in January, previously heard the most recent estimate has put the projected overspend at £700m over 20 years.
Mr Hutchinson conceded that the "fatal flaw" in the scheme design should have been picked up.
He said he had given "too much credence" to the the findings of the economic consultants Cambridge Economic Policy Associates or CEPA who set the tariff levels.
"Looking back you can only say: 'Yes, CEPA should have picked it up, and we should have picked it up and it should have been picked up throughout the scrutiny process'," he said.
