The ad says 8,000 acres, but the land is more like 9,000, and it's at least 10 miles from 1600 Pennsylvania Ave. But poetic license hardly mars this polished ad, one of seven drawn up for Fairfax County's ambitious half-million-dollar campaign to attract big business by promoting the county's proximity to the nation's capital.

A bigger problem is that while the county's salesmen - the Fairfax Economic Development Authority - are drumming up business, the county's nuts-and-bolts men - the land planners - say there isn't rom to put it all.

Sam Finz, Fairfax County's deputy executive for planning, estimates that the county now has fewer than 10 prime sites for big organizations wiht more than 500 employes, the kind business the development agency's ads aim to attract.

It could take two or three years to replan and rezone land to relocate a major business, Finz says, longer than many companies want to wait.

Only 4.3 per cent of the county's 406 square miles is zoned for commercial use and much of that, in the western part of the county near Dulles International Airport, lacks adequate roads, according to Finz.

The county must take "serious steps" forward in replanning and rezoning, he said, if it is going to accommodate the business that county developers hope to attract.

The first of the advertisments, two full pages in the Wall Street Journal at a cost of $26,000, will begin to appear June 29. The headline will read: "The advantages of locating your company amidst the greatest concentration of power and influence in the free world."

Smaller ads, aimed at large corporations and professional associations, will follow in the Journal, Business Week, Forbes. The New Yorker and Dun's Review over the next month 18 months.

The campaign, which Fairfax gave the development agency $559,000, is intended to attract business that will reduce the county's disproportionate tax burden on homeowners.

"We're not out to knock anybody: we're out to show our strengths and do it in a dynamite way," Lee C. Shur, a Gulf-Reston executive and an EDA commissioner, told about 50 state and local politicians and businessmen yesterday at the presentation of the campaign at Westgate Industrial Park.

"This is going to be the most sophisticated campaign in the country for getting business."

But finding good sites remains a problems, Finz said that of the county's most desirable land for business growth, about 65 per cent is not planned or zoned for commercial use.

Fairfax Board Chairman John F. Herrity has called the construction of lanes paralled to the Dulles Airport Access Road the key to the county's major commercial development. The road would make accessible vacant land along a 10-mile corridor, which Herrity envisions developing like Montgomery County's I-270 corridor.

The EDA's long-range goal is to have business contribute 25 percent of the county's tax base and provide 73,000 new jobs by the end of the century. Business now contributes 14.3 percent, down from 16.8 percent in 1972.

"Perhaps the goal is a little unrealistic," said EDA director David Edwards. "But that may not be unappropriate. We need a stiff goal to accomplish anything at all."