THE design of public housing has come a long way since the utilitarian Mark 1 blocks of flats erected in 1957. The latest building line is the Harmony block, which has replaced the previous Trident blocks as the standard design. Named by the previous Governor, Lord Wilson, to symbolise life in estates where people live in harmony with each other and the environment, the design was conceived in 1987. The initial tender was announced in October 1989, and the first building was completed in Tin Shui Wai in 1992. The labour shortage in the mid-1980s was causing considerable problems and on-site delays, and one of the aims of the new design was to simplify the construction procedure and to avoid reliance on on-site labour. The other aim was to improve the quality of the flats. This was done with more standardisation and flexibility. There are three standard Harmony types, each of which has a modular design for flexibility in the flat mix. Once the floor layout of a particular building is fixed, it is repeated on every floor. The flexibility comes from the modular design. Because of the standard-size units, a three-bedroom flat is identical in layout to a two-bedroom flat, with an extra bedroom. The same area can be used for two single-bedroom units. Permutations of the modules allow 10 options: in Harmony 1, a 39-storey cross-shaped tower. Harmony 2 is a 37-storey Trident-shaped tower, while Harmony 3 is a 27-storey building whose flexible Y-shape is designed for height-restricted sites and restricted site areas. Harmony 1 is the most commonly used of the three designs and will be the standard building for future public housing on north Lantau near the new airport. ''We have had feedback from contractors, tenants and project teams,'' said Chris Gabriel, the Housing Department's chief architect. ''As a result, we have upgraded the kitchen by improving the size of the sink basin and food-preparation work top. ''In the bathroom, we have replaced the bath, which many people said was too small, with a shower.'' Locks on front doors, which proved to be too complicated, were also being changed, Mr Gabriel said. The latest addition to the Harmony series is an annex which can be added as an extra wing to any of the other blocks. The annex was designed to accommodate the increasing demand for single-person accommodation and consists entirely of one-bedroom flats. ''Feedback on the single-person flats has been positive,'' he said. Standardisation has helped the contractor. Doors and windows are manufactured by an approved manufacturer and delivered on site complete with handles and locks. Dry-wall systems are bolted into place to form partitions, and staircase units are precast off site. This means less wet concrete work on site, and the result is a cleaner, safer construction site. Large panel steel formwork is still used for the main construction work, but the aim is to increase precasting work wherever possible. The aim is to precast 75 per cent of the facade units, although only 60 per cent is being precast. ''Next year, we will target 100 per cent, although site restrictions may prevent us from reaching this figure,'' said Mr Gabriel. Only Harmony blocks are being constructed for rental projects now, and smaller versions are being built for the Home Ownership Scheme. Although Mr Gabriel does not expect any major changes in design or materials in the immediate future, he says they will continue to evolve, for example by the introduction of more off-site construction, and general improvement in construction methods.