The best site for relocating Fairfax County's cramped seat of government is a 266-acre tract one mile west of its present location in Fairfax City, an official study subcommittee has decided.

The group's recommendation is likely to be accepted by the full relocation committee, its chairman, Reston attorney Michael S. Horwatt, said yesterday. If that happens, the recommendation would then go to the Fairfax Board of Supervisors for final decision, after a public hearing, probably sometime in October.

The site, called the Smith-Carney tract, is between Rtes. 29-211 and I-66 not far from where a regional shopping mall is scheduled to be built.

The site was the least costly of 10 alternatives that had been analyzed by the relocation committee. The net cost to the county if it moved to the site would probably be about $51 million through the year 2020, according to the analysis. If the county stayed at its present location - indowntown Fairfax City - the cost in rental of extra space would range from $72.6 million to $157 million, according to the analysis.

Horwatt, who also heads the subcommittee that settled on the cheapest alternative, said relocation of the county seat would not mean an entire "uprooting" of all offices from Fairfax City to the new site. "The Massey Building (the tower holding many administrative offices) would still be used," he said.

Earlier this year, the supervisors accepted the full committee's recommendation and decided not to relocate the courthouse at any new county seat. The new court facilities will be built, as scheduled, not far from the Massey Building.

The county has been considering relocation because of a shortage of expansion room at the site of the present office complex. The county has been forced to rent the equivalent of 80 percent of the Massey Building's space at 15 locations in the city.

In March, the full committee narrowed its choices to three sites - the Smith-Carney tract, a location one mile farther west, in the community of Pender, and the southeast quadrant of the Chiles tract at the Capital Beltway and Rte. 50. The supervisors asked the committee to take a more detailed look at the Smith-Carney and Pender sites and also the northeast quadrant of the Chiles tract.

The subcommittee, which completed its work earlier this week, gave the Smith-Carney tract its highest recommendation, found the Pender site "acceptable" and the northeast quadrant of the Chiles tract "unacceptable."