Growth in Fairfax County could far outstrip the predictions of some top county planners by the mid-1980s, a new, private study of the Washington-Baltimore housing market indicates.

If correct, the study's findings mean county officials may be underestimating future demand-encouraged by the county's own largely pro-growth policies-for expensive new services.

It also could mean the disappearance of the few large areas of open space as developers subdivide the landscape.

The study, prepared by American University urban affairs expert Maury Seldin for an unidentified major multinational company, says there will be a market in Fairfax for almost 60,000 new houses between 1978 and 1985. According to Seldin, most of the demand-70 percent-will be for single family dwellings, which house the biggest families, consume the most land and require the most services.

If the demand is met, Fairfax's population between 1978 and 1985 could grow by 185,000, or more than 50 percent faster than the county's current projections, which have been approved by the Washington Metropolitan Council of Governments.

If development proceeds as Seldin forecasts-and he says that "Fairfax is pretty well opened up. . . constraints are minimal"-most of the county's few large areas of open space would be converted to subdivisions and there would be a continuing, even growing, demand for expensive new schools, libraries, police and firefighters and roads.

Already, in the face of surging development in southwestern and northern Fairfax, the county government is mobilizing its resources to build a $125 million road-called the Springfield Bypass-that would are from Rte. 7 near Herndon and Reston through the Burke-Springfield area and extend to Franconia south of Laexandria.

Fairfax is counting on federal assistance, but if such aid doesn't materialize, the county would have to pay for a substantial part of the road's costs.

Though studies of growth costs generally say concentrated development is the cheapest kind to service, the pattern in Fairfax is a crazy quilt of urban sprawl.

Subdivisions of single-family houses now stand in staggered ranks in the once largely open Difficult Run watershed from near Fairfax City to the Potomac River in Great Falls.

Similar scattered development, product almost entirely of market forces, is spreading through Great Falls and the Occoquan, two areas where the Board of Supervisors considered-and later abandoned-controls to limit growth.

"There isn't much you can do." Chairman John F. Herrity said. "Controlling growth with legislation is like trying to empty the ocean with a teaspoon. And now the courts have taken away our teaspoon."

While Seldin, in his study, sees suburban Maryland having an upsurge in development, he singles out Fairfax as the No. 1 growth center.

His reasons: Fairfax has more residents who are considered prime home buyers, more families with an "effective buying power" over $25,000 annually, fewer environmental controls and other restrictions that put a brake on growth, and is attracting more jobs than its suburban neighbors.

To buttress his forecasts, Seldin cites these figures:

One-third to one-half of all heads of households in Fairfax are in the "prime buying years"-54.3 percent over 25 years of age and 37 percent over 35 years of age.

Sixty percent of Fairfax County's families have an effective buying power of $25,000 or more, compared to 50 percent in Montgomery County and 35 percent in Prince George's.

"Facilities and enironmental constraints are serious in Montgomery," but "are minimal" in Fairfax.

Fairfax increased its jobs at a rate of 4m.4 percent between 1971 and 1978, while Montgomery's rate was 29.5 percent and Prince George's was 22.8 percent.

Seldin's study-with its optimistic forecast for housing growth in Fairfax and, to a lesser extent, elsewhere in suburban Washington-reportedly played an important role in helping his client company decide to enter the residential construction market in the area.

Based on these factors, Seldin sees the housing demand in Fairfax averaging about 8,400 units a year from 1978 to 1985. At present, Fairfax is building about 8,000 units and adding about 25,000 people annually.

Some county planners-especially those in the budget office-have forecast a slowdown in this fast pace as high interest rates and other anti-inflation policies take hold. But other planners are not so sure.

"I have a general feeling that Fairfax is going to be pretty well insulated from the national trend [toward a downturn in housing production]," said Mary Elizabeth Noe, director of the county's Office of Research and Statistics. "I think you can see a softening, but still there is a temendous demand for housing, and a fantastic demand for resales."

Fairfax Deputy County Executive Samuel Finz, who is in overall charge of planning and development, said Seldin's estimates seem to be borne out by the pace of growth under way and indicated by plans.

"I don't think it [the growth] is gooing to let up," he said.

Fairfax's growth surge is coming at a time when its residents, especially those who no longer have school-age children and who live in settled areas demanding few new services, are growing increasingly hostile to tax increases.

But growth-especially the common practice of widely scattered development-demands that expensive servies be provided.

To pay for the services, the county is trying to attract more business-on the theory that commerce and industry are a net gain fiscally-but nonetheless the county's assessable tax base is becoming increasingly weighted toward residences.