The Fairfax County Board of Supervisors and Herndon officials have agreed unofficially to proceed with a controversial land exchange proposal that could give the town nearly $500,000 a year in added tax revenues.

Supervisor Nancy K. Falck (R-Dranesville) said the other eight members of the board had agreed in a closed meeting to support revised terms of the land swap. Before the swap is completed, it must be approved by the Board of Supervisors in a public meeting, the Virginia Commission of Local Government and the Fairfax County Circuit Court.

Under the proposal, the county would give Herndon about 100 acres of potentially valuable industrially zoned property south of the town's border in exchange for 72 acres of residentially and commercially zoned land along Stuart Road and south of the Dulles Toll Road.

The proposed agreement also calls for Herndon to relinquish any claims to annex any more land in Fairfax County for 18 years, and to nullify the property exchange if the town obtains city status under Virginia law during those years.

It took county and town officials nearly two years to develop the land swap. Herndon officials had strongly opposed the county's earlier offer because one condition would have prohibited the14,500-population town from applying for city status for at least 25 years.

Herndon Mayor Richard C. Thoesen called that condition a "loss of political face and town pride."

Thoesen said the latest proposal, which was supported unanimously by the Herndon Town Council in February, "very fair . . . . The county wanted to be assured we wouldn't take land away from its real estate coffers."

Thoesen viewed Herndon's economic future as healthy as a result of a recent boom in commercial and light industrial development inside the town's boundaries. Fairfax County officials were previously upset at the prospect losing approximately $2.7 million a year in tax revenues from Herndon if the town is elevated to city status.

Town residents and businesses now pay county taxes as well as Herndon taxes and receive county services in return. If Herndon becomes a city, all local revenues would go to the city, and it would have to provide most of the services that the county now provides.

Fairfax supervisors recently approved a huge commercial development on 94 acres of the 100-acre site, which, if the land swap is approved, would give Herndon about $500,000 a year in tax revenues by the mid-1990s.

The development, called Worldgate Office Park Inc., will consist of nearly a dozen six- to seven-story buildings, which will house two hotels, restaurants, a health club, shops and offices.

Board members said they were satisfied with Herndon's promise not to try to annex more county property for almost two decades, and to give back the valuable chunk of property to the county should the town obtain city status.

"We wouldn't want to be an impediment to the health and well being of the Town of Herndon," said Falck in an interview. "If you're a resident of Herndon, you're a resident of the county also. There is no animosity there."

Thoesen said the town had no intention of seeking city status in the near future.