Fairfax County could save $80.6 million during the next 35 years by building a new government center near Fair Oaks Mall and ending its practice of renting office space in Fairfax City, according to a new county study.

The study is the latest salvo in a decade-old battle over the proposed $71 million complex, which would be located at the intersection of I-66 and Rte. 50.

Opponents of the government center have called it a "monument to bureaucracy" and an "edifice complex" with little popular support and have blocked attempts to put the issue up for a vote in a bond referendum.

Supporters of the complex argue that the county government has outgrown the current 12-story county administration building and spilled into leased offices in Fairfax City, costing the county about $5 million annually in rental costs and expanding the city's tax base, rather than the county's.

The overflow into private office space has grown as the county's bureaucracy has more than doubled, jumping from about 3,000 employes in 1970, shortly after the current building was completed, to more than 7,000 today.

A blue-ribbon commission endorsed the idea of a new government complex in 1978, and the following year the county spent $4.1 million to buy a 183-acre tract of land for the site west of Fairfax City. Construction has never begun.

Last month, the county board cut $14 million from County Executive J. Hamilton Lambert's fiscal 1986 budget that would have paid for building roads and water and sewer lines at the Fair Oaks site.

The new report, prepared by the county staff, says that Fairfax will spend about $151 million until 2020 if it continues to rent office space in the city. The study says that continued leasing will cost the county more than either erecting a new government complex or expanding the current 48-acre site.

"The relocation of the county government center . . . . is a multimillion-dollar savings to county taxpayers," said Supervisor James M. Scott, a Democrat who requested the study.

Scott called the relocation "a real good deal" and said he hopes the county board votes to put a bond issue for the complex on the ballot this year or in 1986.

Board Chairman John F. Herrity, a longtime critic of the proposed complex, said the study neglected to consider the benefits of selling the 183-acre site near Fair Oaks Mall and using the proceeds to fund an expansion at the existing site in Fairfax City. He said selling the land could produce more than $40 million for the county.

Scott said expanding the current government site would further clog already crowded roads leading there, making it nearly inaccessible.

Herrity, a Republican, also said putting the government complex to a referendum would jeopardize the board's plan to propose a bond issue of up to $80 million for road improvements on the ballot this fall.

"Our priority in this county is roads," he said, "and we're not going to get that priority mixed up with any boondoggle like the county government building."