Fairfax County planners have concluded that at least 11,600 new apartment units will be needed in the county by 1995 to meet the housing needs of the jurisdiction's expanding population and changing life-style patterns and demographics.

Such large-scale apartment construction would mark a major change in development patterns in Fairfax, where the overwhelming majority of residential construction in recent years has been single-family detached homes and town houses.

At the moment, only 600 acres of land in the county is designated for multifamily or apartment development, according to a new study by the county's comprehensive planning office. The report, scheduled to be given to the county planning commission next Thursday, said major changes should be made in the county's land-use plan to set aside additional tracts for apartment construction. The planners projected the county's 1995 population at 734,240, up from the current 668,290.

The report echoes the concerns of many county developers, who have called for construction of garden apartments to house mid-level management and support personnel moving into Fairfax, where the median price of a house now tops $130,000.

The need for more apartments is not only a result of the county's continuing economic boom, but also a reflection of the increase in the number of single individuals, both young and old, living alone and wanting low-maintenance units coupled with such amenities as swimming pools and health facilities.

According to the Fairfax Office of Research and Statistics, the county has experienced a 250 percent jump in the number of one-person households and a 103 percent increase in the number of two-person households in the last 15 years. Those figures dramatize the growing need for apartments, according to county officials.

"Ideally, market forces will be responsive to the demand for multifamily residential development," the study said. "However, the residential development industry is subject to many constraints imposed by outside forces. The planning policies of local governments, for example, can be a significant factor in stimulating or retarding the residential market."

Multifamily development has not kept up with potential demand because of the high cost of land, the shortage of land planned for multifamily use and density limitations imposed by the county's zoning laws, according to the study. The report said Fairfax officials "must address the factors" they can control, such as planning and zoning.

The study calls for increased apartment construction in all sections of the county, from western Fairfax, where large tracts might be available, to neighborhoods inside the Capital Beltway, where apartments might be built on some remaining vacant land or on underused commercial tracts.

In recent years, a lot of the land that might have been suitable for apartment construction has been used by developers for condominiums and town houses, which provide developers with a much quicker return on their investment.

The study noted that developers would prefer to build garden apartments at a density of 20 to 30 units per acre, up from the existing limit of 16 to 20 per acre. Mid-rise and high-rise development would be more attractive to developers and investors if they could build 30 or more units per acre, the study said.

Those densities, which are high for Fairfax County, are much lower than those allowed in Arlington County, where high-rise developments have ranged from 46 to 228 units per acre, according to county statistics.

The study said that the designation of 600 acres for apartment construction "does not allow the market to operate freely" and that it is "necessary to provide the opportunity and incentive" to developers to build apartments by finding additional sites.

Several months ago, Fairfax County supervisors appointed a study group to tackle the shortage of affordable housing in the county, a problem that goes hand in hand with the lack of available apartments. Developers and the Fairfax County Chamber of Commerce also are seeking solutions to the high cost of housing.

The report recommends that new apartments be built when possible near Metro subway stations, near bus lines or convenient to major roads.

The study also said multifamily development should be "compatible" with neighboring housing or serve as a transition from one land use to another.