Norman Foster's architectural firm, defending its design for the West Kowloon Cultural District yesterday, insisted that the plan's green features were not responsible for HK$4 billion in cost overruns. Spencer de Grey, a senior executive of the Foster team, which is in charge of the final design of the 42-hectare arts hub, said he did not know why the hub authority said last week that underground facilities and other green features would add an extra HK$4 billion to the costs. 'A green approach, including an underground car park, has always been an integral part of our proposal,' de Grey said. 'We have a good track record of taking costs very seriously.' He denied that underground roads, car parks and the waste and water recycling systems were responsible for raising the construction costs of the HK$21.6 billion project by HK$4 billion, as the authority's chief executive Michael Lynch told lawmakers on Friday. 'You must ask the authority [how the sum was calculated],' he said. He maintained that putting traffic, parking and servicing below ground was a flexible way to use space. Lynch had said the financial situation of the arts hub project was vulnerable because of high inflation, surging construction costs and low investment returns from the government's HK$21.6 billion endowment in 2008. The authority had said, in a paper submitted to the legislature, that the economic environment in recent years was very different from the long-term assumptions used in the analysis by its financial consultant in 2006. For example, the construction costs in 2010 were 49 per cent higher than estimated. Sin Chung-kai, board member of the West Kowloon Cultural District Authority, said neither Foster's nor the other two competing designs fitted with the HK$21.6 billion budget. 'They went over the sum to different degrees,' he said. 'To be fair, we didn't make it a requirement that they must fit the figure. Each of them had their own budget.' Sin said the higher costs of building underground facilities, a key element of Foster's design, were not taken into account in the 2006 assessment, since the design came afterwards. 'What we now should do is to sort out ways to increase income,' he said. With lawmakers saying they would not easily approve more money for the authority, the executives proposed new ways to raise funds. They will review the possibility of building the more expensive mega-venues at a later stage, explore sponsorships and naming rights, and reduce the number of car parks which are underground. Foster's team is consolidating the final design with elements from the other two architect teams, taking reference from public feedback. The ultimate design will be released for the last round of public consultation, which starts at the end of September and lasts for a month. It will then go to the Town Planning Board.