Fairfax County, in what planning officials describe as a major shift in regional growth patterns, will become the most populous locality in the metropolitan area by 1990, according to revised population projections.

At present, Fairfax, whose 1977 population was 573,200, is smaller than the District of Columbia (691,500) and the two big Maryland suburds, Prince George's County (675,000) and Montgomery County (593,500).

But the revised projections that look ahead to the year 2000 show that by 1990, Fairfax should have a population of 781,000, pushing it past all its now-bigger neighbors.

A dramatic falloff in migration to the area, smaller households and sewer restrictions have all combined to put a brake on growth in the Washington area-a trend that was first detected in the early 1970s and confirmed with the 1976 round of population predictions.

Despite all these dampers, Fairfax is in the midst of one of its more rapid periods of growth-adding about 25,000 people yearly to its population, more than suburban Maryland and the District combined.

After early, abortive efforts to control growth, the Fairfax Board of Supervisors has in effect let the rate be controlled by market forces.

The new projections, prepared by the locatities and approved yesterday by the planning commitee of the Metropolitan Washington Council of Governments, show all the major jurisdictions-Fairfax included-growing at slower rates than were anticipated during the last round of projections in 1976.

But because the revised figures chopped 153,600 people off Prince George's projected increase by 1990 and only 71,100 off Fairfax's, the Northern Virginia suburb is seen moving into the No. 1 spot.

"This represents a major shift in population patterns," said Elizabeth B. Davison, a planner for the Maryland-National Capital Park and Planning Commission who headed the COG forecasting subcommittee. Her assessment was backed up COG's planning coordinator, Jane P. Rogers, who oversaw the updated analysis.

While growth in the suburbs continues to slow down, the so-called exurbs-Loudoun and Prince William counties-are expected to add more people than even anticipated in the bullish projections of 1976. By 1990, Loudoun is now seen growing from 58,500 to 138,000 and Prince William from 144,200 to 290,000.

The revised figures confirm earlier projections about the District that found the national capital falling from its present position as the population center of the region. In fact, the new figures project the District losing ground faster than had been anticipated.

For example, the 1976 figures saw the District growing to 771,500 in 1990 and 793,300 in 1995. The 1978 figures see the city reversing a long-term population decline by 1980, but project a slower growth rate to 1990 and then declinng again-to 720,000-by 1995.

It the new projections are borned out, the District will be only the fourth largest jurisdiction in the region by 1995, behind Fairfax, Prince George's and Montgomery, in that order.

Davidson said Prince George's was expected to grow at a far slower rate because of the more clearly emerging impact of the county's decisio to discourage apartment construction and encourage the building of single-family houses. The policy grew out of former county executive Winfield M. Kely Jr.'s "new quality" program, aimed at giving the county a glossier image.

According to Davison, the emphasis on single-family houses is threatened by the rising interest rates for mortgages. Already the current area rate or 11 percent exceeds the 10 percent ceiling of Maryland's usury law-and lending institutions in both Prince George's and Montgomery are no longer granting loans.

The Maryland legislature is expected to raise the rate at its next session, but the effective date may be delayed until later in the year and the new ceiling may itself be exceeded before interest rates level cut.

In Fairfax County, the latest of a series of actions that have enhanced the growth atmosphere came Monday when the Board of Surpervisors decided to kill a plan to limit population densities in the environmentally sensitive Great Falls and Occoquan Reservoir areas.

"The opportunities in Fairfax have hardly been touched," said prominent zoning attorney John T. Hazel Jr., whose Burke Centre development in southwestern Fairfax and record sales for the month of November despite rising mortage interest rates. CAPTION: Picture, ABANDONED AGAIN - This burned-out roof structure is evidence of dangerous conditions at abondoned cement plant in Rockville where six itinerants lived until one died recently. The other five pulled out. By Gerald Martineau-The Washington Post, Graph, Metropolitan Population Growth, By Richard Furno-The Washington Post