GOVERNMENT House is set to become a suit-free zone on December 17 as the Pattens back a ''Dress Down Day'' organised by the Community Chest. Lavender Patten, the Chest's president, yesterday welcomed the chance to return to a more casual way of dressing and predicted that the Governor, who has already done away with his plumed hat, would join in. Chest vice-president David Davies said he would like all the territory's six million inhabitants to join in on ''Dress Down Day''. People would be allowed to go to work in casual dress after making a donation to the charity. Firms throughout Hong Kong, particularly the large corporations in Central, Tsim Sha Tsui and the new towns, are being asked to co-operate with the fund-raising day. Possible obstacles may include the exclusive clubs and hotels, which will be asked to relax their dress rules. After she launched the Community Chest's 1993/ 94 campaign which aims to raise a record $150 million, Mrs Patten said: ''It will be very nice to dress down. ''My natural inclination is to look fairly casual. ''People in Hong Kong, particularly the women, do dress up, so it will be a nice change. ''I just hope I haven't got any really formal events that day - I shall have to go back and check my diary.'' She predicted that the Pattens would don their ''Saturday clothes'' of trousers and sweaters. The Hongkong and Shanghai Banking Corporation and Hongkong Telecom have already pledged their support to the ''Dress Down Day'' and Chest organisers are hoping that on December 17 the sight of someone in a suit will be rarer than a fall of snow. But it will be tough to reach the overall target of $150 million for the year, which would see the Chest break the $1 billion mark in its 25th year, Mr Davies said. ''Last year we were well supported by the Bank of China walk which raised $19 million and helped us bridge the gap and meet last year's target,'' he said. ''The demands of the agencies we support increase each year and we always start off with this lump in our throats about whether we can make it.'' Other fund-raising events planned for the silver jubilee year include a Government House ball and special MTR tickets, together with the annual events such as Walks for Millions. Mr Davies also announced that the Chest has appointed a new executive director, Darwin Chen, to start work in December. Mr Chen, who has worked in the civil service for 30 years, most recently as Director of Buildings and Lands, will take over from the interim chief and former executive director Denis Bray, who took on the post again when William Lam retired in June.