BUILDING contractors on the $12 billion passenger terminal at Chek Lap Kok are claiming an extra $1.6 billion from the Airport Authority - just nine months after a $1.9 billion payout for other added costs. Sunday Money understands the claims, as a supplemental agreement to the initial contract, have just been lodged with the authority by BCJ, the Britain-China-Japan joint venture responsible for construction of the terminal. They cover design changes, schedule variations and delays which have arisen since a previous supplemental payment was agreed last September. 'The group got the building weather-tight just about on schedule while getting other elements ahead of schedule. It is doing a good job, but there is a cost to that,' an on-site source said. BCJ consists of Amec and Balfour Beatty from Britain, Kumagai Gumi (HK), China State Construction Engineering and Maeda of Japan. In September, BCJ and AEH - the building services installer - were paid $1.9 billion to settle outstanding construction wrangles on the project. The payments angered legislators who demanded that senior authority executives give a full explanation for them. The wrangle recently resurfaced after suggestions the claims were paid despite little supporting paperwork setting out a precise cost schedule for the delays and other problems suffered by BCJ and AEH. There also have been allegations the authority is being 'held to ransom' by contractors demanding millions to finish their contracts on time. The authority's corporate development director, Clinton Leeks, said the initial supplemental agreements were made at an extremely high level in both the authority and the contractors. Meanwhile, the $2.1 billion northwest passenger terminal extension, given the go-ahead just over a year ago, may be roofed in concrete instead of steel like the main building. Three groups - Paul Y-ITC, Shui On and BCJ - submitted tenders, with concrete the cheapest option. BCJ, which would continue using steel, was the most expensive of the three. But the authority says all bids were overpriced and may itself manage the project and subcontract the work into packages covering steelwork, concrete, glass and associated materials. It is not known when the authority will announce its decision.