Fairfax County, in what one highranking county official called "a tremendous deal," has paid $3.9 million for 183 acres west of Fairfax City as the site for its new government seat.

The land is located in the fastgrowing western portion of the county on Rtes. 29-211, near the intersection of Interstate Rte. 66 and Rte. 50. It was purchased for about $21,000 as acre, which real estate salesmen say is a fraction of its potential worth.

The purchase agreement, signed Friday, apparently was prompted by a compromise approved by the Board of Supervisors in a closed session early last Tuesday. The agreement would let the owners keep 80 acres of the 263-acre site for other development.

Creation of the government center, along with completion of the nearby Fair Oaks Regional Shopping Center, now under construction, would make the 80 acres extre?ely valuable to the Owners, Benjamin Smith Jr. and Jack W. Carney. If the land is rezoned from its present residential use to commercial or industrial use, it could be worth up to $100,000 an acre or more, based on comparable land deals in the area.

Smith and Carney had another inducement to sell their land to the county as cheaply as they did, officials say. Because they sold the land while a county-filed condemnation suit was in process, officials say the owners can avoid any capital gains tax on profits from the sale of plowing them into another project, just like a home seller who bhys again within a year.

Fairfax did well in the deal too, in the estimation of high county officials who took part in the negotiations.

"It's a bargain," said one official who asked not to be identified. "The county saves about a million dollars," the official said, when comparing the final deal to Fairfax's original offer to buy the total 263 acres of the tract.

Furthermore, the officials said, the county ended up paying no more per acre than it had offered in its original proposal. It was the owners' rejection of that proposal that led to the county's condemnation suit.

Though the county now has the land on which to build a new, roomier governmental center to replace the crowded Massey Building complex in Fairfax City, actual construction will depend on voter approval of a bond referendum. The cost could run to$50 million depending on how large the complex is. But before there is a referendum, the county must decide just how many offices would be moved to the new location, and what the new center will look like.

Fairfax lawyer Michael S. Horwatt will head a committee to draw up a master plan for the center. Based on that plan, there will be a national architectural competition for the design of the center.

If the center is approved by the voters, it would be built in phases, according to county officials. Construction could begin as early as 1981 and would extend to 1985.

County officials say they have no intention of moving the county courts from Fairfax City nor do they intend to leave the county-owned Massey Building unused.

A new courthouse building is under construction and, officials note, it would take a special election to move the courthouse itself from Fairfax City.

Fairfax City has been the seat of the Fairfax County government for more than 175 years. County leaders have been becoming increasingly interested in relocating to a new center, preferably one on land within the county. A relocation study headed by Horwatt contended that such a move permit premit the county to reap tax benefits from adjacent commercial and industrial development - an advantage presently enjoyed only by independent Fairfax City.

A new location, the study also said, would relieve the cramped conditions that exist around the Massey Building. The 11-story administrative tower had all of its space filled soon after it was opened in 1969.

On or near three major east-west highways, the tract is easier to reach than the Massey Building. If the future Springfield Bypass extends northwest from Rte. 123, the site would be easily accessible from population centers in the southern and northern parts of the county.

The county purchased the tract with $2.5 million that was borrowed from a solid waste disposal fund and $1.4 million from the emergency public reserve fund. CAPTION: Map, no caption, By Dave Cook - The Washington Post