THE Government and the military have denied a report that Hong Kong has been asked to find an extra $480 million on top of its normal defence budget to pay for the withdrawal of the British Garrison. They said the costs were included in the Defence Cost Agreement (DCA) signed in 1988. The withdrawal cost is believed to be between $250 million and $300 million. A military spokesman said under the DCA, Hong Kong would bear 65 per cent of the additional costs with the British Government finding the rest. A government spokesman denied that Secretary for the Treasury, Donald Tsang Yam-kuen, had claimed the British requested an extra $480 million. He was reported in one newspaper as being ''shocked'' with the British request and having to persuade them to change their minds. Mr Tsang declined to comment. The Sino-British Joint Liaison Group's sub-group on defence land will meet again today and tomorrow following a four-hour session yesterday. Leaders from both sides said the meeting mainly dealt with technical issues. It is understood that China has yet to agree to sign an undertaking that the People's Liberation Army will return to the future Hong Kong government any unused military sites. Another issue is understood to be China's insistence on a bigger naval site than the one provided for on Stonecutters Island.