FLAT prices at Belvedere Garden in Tsuen Wan have increased steadily since the end of 1992, avoiding the end-of-year trough recorded in other residential developments. The average price of a flat at the estate in December was $3,350 per square foot compared to an average price of $2,723 in December, 1992. Belvedere Gardens, which is about a 15-minute walk from the Tsuen Wan MTR station, was built about four years ago. Flats range from 500 to 1,000 sq ft and have been popular choices for first home buyers. Brooke Hillier Parker partner David Faulkner said sales figures at the estate were indicative of price trends in the area. ''Sale prices at estates located on the MTR have typically performed better than prices at developments where residents spend more time commuting to work each day.''