TOTALLY self-contained planned communities could be the United States' fastest-growing type of residential real estate over the next decade. About 30 projects have been initiated in the US over the past 10 years and an indeterminate number are now on the drawing board. The increasing interest surrounding planned communities has come as municipalities and builders have searched for an alternative to the suburban growth which has blanketed vast areas of the country since the 1960s. The new planned communities attempt to develop small towns that include not just homes but also jobs, retail and recreational facilities, all within walking distance or via local public transport. One community which is currently filling up is Rancho Santa Margarita, about 50 miles south of Los Angeles. Although the community is still about 10 years away from full development, so far it has attracted about 17,000 people living in about 6,000 homes. A feature of planned communities is the mixing of different types of home, including subsidised housing for the poor, to attract a wide social spectrum which will create a genuine community. Homes range from about US$350,000 for a 3,000-square-foot home to a rental of around $600 per month for a non-subsidised one-bedroom apartment. Of main importance in the efforts to create a balanced community, Rancho Santa Margarita has attracted employment for more than 3,800 people and 70 per cent live in the community or within 15 kilometres of it. The loss of a major employer, a division of Hughes Aviation, meant the loss of 500 jobs but this has been balanced by retailers who have arrived to serve the local community. Theoretically, planned community developments have a range of practical and social advantages. For example, bringing housing and jobs together eliminates a lot of the daily long-distance commuting that imposes increasingly high infrastructure costs on suburban communities. Also, commuting times of more than two hours a day are common on Southern California's crowded freeways. It is generally assumed that people benefit from an improved sense of community resulting from living together with common open spaces and with a wide social diversity within the community. Not surprisingly, the concept of planned communities has been around for some years. Numerous more or less enlightened companies built ''company towns'' for their employees in the middle of the last century when the relationships between living conditions and work performance started to be examined. These include areas such as Kew Gardens in the New York City area of Queens. The grand urban planning of the 1960s also welcomed the concept of planned communities. Reston, Virginia, outside Washington D C, is a vast self-contained community which, after many years, completed its central area in 1991. This is just one of the 16 communities into which the federal government poured millions of dollars during the 1960s. However, 12 of these subsequently defaulted on their loans with the result that any planned community development now comes very much from the private sector. Ecological concerns are now paramount for developers attempting to launch new planned community projects. Sterling Forest, planned for a 6,885-hectare tract, an hour's drive north of New York City, has attracted the condemnation of environmental groups. ''It's a fantasy to expect that you could drop a city of 35,000 people in the last forested open space in the metropolitan region without forever destroying the ecological benefits it provides,'' said Mr Lee Wasserman, director of the New York State Environmental Planning Association. The developer, Sterling Forest Corp, has now obtained community approval for its plans but is under no illusions about clearing the environmental hurdles. According to community relations spokesman Mr David McDermott, the ''preliminary application'' process could take as long as 18 months and will almost certainly involve some cuts in the intended scope of the project. Sterling Forest's planners will be arguing that the plans they are putting forward are more likely to preserve than destroy open space over the long term. ''Orange County projects a need of over 5,000 new households in the next 20 years. ''If you house them in the typical pattern of large lot suburban development it would require virtually all available farm land and open space,'' said Mr Robert Thomson, chairman of Sterling Forest. But Sterling Forest plans to use just 18 per cent of its available space for its 14,200 houses, plus a further six per cent for commercial property. The falling northeastern property market of the last few years has not affected plans for the project. ''By the time we have finished we expect to have gone through at least three cyclical swings in the property market,'' said Mr McDermott. ''If anything, we are glad the market has been so bad recently. This has given us the space and time to make sure we get our plans right.''