When American Telephone & Telegraph announced plans last year to move its regional office from the District of Columbia to Fairfax County, employe Dan Kennedy saw an opportunity to sell his house in suburban Maryland and move close to the new location.

But even with financial assistance -- which giant AT&T has offered him -- Kennedy has found the move out of reach. Faced with current housing prices and the prospect of giving up his 8.25 percent mortgage in Maryland, Kennedy will be stuck with a three-hour round-trip commute when the new office opens in three months.

"I don't like the fact that I will be spending more time with my carpoolers than with my kids" Kennedy says, "but I can't see myself eating Cheerios for dinner and taking my vacation in the backyard."

Nor is Kennedy alone. Of the 300 to 400 AT&T area employes eligible for company-assisted moves, company officials say, only about 70 have set out to buy homes near the Oakton plant, 25 miles west of downtown Washington.

Other major fimrs like Mobil Oil and Sperry Corp., lured to Fairfax by and aggresive county recruiting campaign that touts the area's proximity to Washington, are beginning to face similar difficulties, executives say.

"We have seen problems in the (home) buying end," says George Reiger, a spokesman for Sperry, which is completing a $4.5 million research and design facitlity in Reston next spring. "At least one fellow is having serious reservations about coming here because of the mortgage situation, and there may be more."

County officials are quick to discount any impact such a trend might have on growth-oriented Fairfax, which commands about 40 percent of the commercial and industrial real estate development in the metropolitan area. t

"For those businesses who have to be here, whether to serve the federal government or whatever, they will have to bring their employes in whether it poses additonal costs or not," says David Edwrds, executive director of the Fairfax County Economic Development Authority.

John F. Herrity, reelected last week to a new term as chairman of the county board of supervisors, also has promised renewed efforts to attract light industry to Fairfax as a way to lighten individual taxpayers' burdens.

None of which sways Kennedy, who found he couldn't afford to buy a house in Fairfax as big as the three-bedroom residence he purchased for $33,000 near Bowie six year ago.

To buy a comparable home at today's mortgage rates, he figures he would increase his monthly house payments from $289 to $700 -- far above what is considered acceptable on his $25,000 annual inrome.

Married and the father of three children, Kennedy says he would rather commute three hours than push his wife into working to supplement his income. " suppose I feel resigned to it, but it's upsetting," Kennedy says. "I feel victimized by the mortgage interest rates. I don't feel any consideration has been given to the average working individual."

Many of his colleagues have chosen to wait out the market, hoping the mortgage situation will improve before the new facility opens its doors in February. Even if it does not, AT&T's offer of assistance -- ranging up to payment of $7,500 to offset higher mortgage interest payments -- will extend until a full year after the planned move.

For employes of firms such as AT&T, which are only moving across town, the answer is relatively simple: follow the company to its new location and keep the old homestead -- a decision which carries with it promises of longer hours on the Beltway and a daily gamble against unsteady gasoline supplies.

But employes of Mobil Oil Corp., which plans to move its U.s. Marketing and refining Division headquarters to Fairfax County next year, face a more difficult dilemma.

Almost all of the 1,000 employes at the new facility are slated to be transferred from the company's New York City offices. Many are homeowners in areas governed by stiff usury laws, and rising mortgage rates are making them wonder whether they will be able to sell their homes at all.

Al Boyer, a relocation counselor who is working with the Mobil employes, says a recent decision by some New York area banks to require 40 percent down on scarce mortgage loans has only made matters worse. "We can't predict what will happen between now and the spring (when the Mobil workers must decide whether to put their homes on the market.) It all depends on the mortgage rates."

The Yeonas Company, one of the area's largest home builders, has seen 13 potential buyers pull out of home contracts for financial reasons in the last two weeks -- a number drastically higher than the four per month it had previously averaged, company officials said.

The Northern Virginia Board of Realtors reports that overall October home sales were down 5 percent compared with 1978, and many realtors say prospects are for a much sharper drop in the near future. Bob Johnson, the executive vice president of the Northern Virginia Builders Association, estimates that sales of new homes in the area are down 15 percent over last year.

"People who stand to be hurt the worst through all of this, and I know it's a cliche, are the young and the elderly," says Johnson. "Everyone else will put off their buting decisions for the next 90 days or so, but after that time we'll go into a sales upswing again.

"It's the psychological factor of the interest rate that is so important. When people know the interest rate is artifically high, those who can afford to wait will do so."