The Fairfax County Department of Consumer Affairs normally spends its time investigating car dealers, merchants, health spas and developers. Last month it took on the giant Woolco discount stores, and won.

The consumer affairs office convinced F. W. Woolworth Co., the stores' parent company, to sign an "agreement of voluntary compliance," promising to lower any overpriced products and refund money to any customers who had been overcharged.

For an agency that, by its own admission, successfully resolves just a little more than half of its cases, it was an impressive victory and one that came as Maryland officials were trying unsuccessfully, under laws that some consider tougher than Virginia's, to win similar concessions. It was only the ninth such agreement that the Fairfax consumer affairs office has obtained since the procedure was created by the Virginia Consumer Protection Act of 1977.

"I was surprised, but then again our consumer affairs folks are usually on the ball. I was pleased they made such a quick settlement," said Fairfax Supervisor Joseph Alexander, who asked the agency to investigate after his wife told him that the price of an extension cord had increased once Woolco's going-out-of-business sale began.

The agency says that in the Woolco case it had the good fortune to be dealing with allegations clearly covered by Virginia laws, charges that easily could be documented, procedures that allowed the company to cooperate without admitting any wrongdoing, and a cooperative company. That, says the department's staff, is not always the case.

The department has a budget of about $965,000 for the 1983 fiscal year. It receives some 28,000 calls and about 4,000 formal complaints a year. That figure has dropped somewhat during the past year because the agency had one telephone line removed to reduce the work load, said Director Ronald B. Mallard.

In the 19 years since its creation, the department has branched into several areas that are unusual for a consumer agency, including landlord-tenant disputes, the licensing of taxi drivers and, most recently, cable television. All of this gives it the potential to become one of the most influential agencies in the county.

It also is charged with ensuring the accuracy of such measuring equipment as gasoline pumps and grocery scales. Seals and red tags are placed on instruments that are not accurate, and the seals are not removed until the devices are fixed. About 20 percent of the 3,960 gasoline pumps checked last year were tagged.

The 35-member agency still is most closely identified, however, with the task for which it originally was created: resolving consumer complaints, usually related to auto repairs, home construction and improvements, or retail stores.

Although the Woolco case received more publicity, it was not that different from other complaints that the agency handles. The investigation was sparked by a handful of inquiries from customers, including Alexander. After rummaging through his basement to find old Woolco advertising inserts, Tony D. Provine, supervisor of investigations, and two other staffers put together a "regular" price list and went on a shopping trip to the three Woolco stores in Fairfax.

They found that in 25 percent of the cases the "regular price" was overstated, Mallard said. Armed with the price list, Provine met with Woolworth's attorneys and, while denying any wrongdoing, the lawyers agreed to sign the agreement and lower the prices of the items cited by the Fairfax investigators.

The Fairfax agency does not have subpoena power or the authority to order businesses to cease-and-desist a certain activity, unlike Maryland consumer agencies.

"It can be a problem," said Mallard. "Maryland has more authority in sheer legal terms than we do."

Yet in Maryland on Friday, a Prince George's County Circuit Court refused to enjoin Woolco from any attempt to boost its regular prices for its sale, set to end Jan. 15. In the court case brought by the Maryland Attorney General's office, Woolco said that the regular prices reported during its sale were accurate.

Fairfax can go through the commonwealth's attorney or county attorney's office to request a subpoena or an injunction, and Provine said that the lack of additional authority has not affected many of the agency's cases.

"It's kind of like asking someone without money, 'Can you have fun without money?! " he said. "Sure you can. Of course, it's a different kind of fun than you would have with money."

Asked about the low number of agreements of voluntary compliance signed, Mallard said: "When we were first granted the authority, we had a dry period" during which the agency was "very conservative in its use" of the 1977 law. He said that the office has become "far more aggressive" in employing the provision, and he expects the number of signed agreements to increase significantly during the next five years.

The Fairfax agency has the potential to become more influential than many of its counterparts because of the range of duties the Fairfax government has given it.

Recently, the county board designated the consumer agency as the sole regulator of the private cable television industry in the county, as well as the futrue operator of county-run cable channels. The agency currently is hiring seven professional and two clerical workers to staff its cable division, which has a budget of $295,000 for fiscal 1983.

Besides handling consumer complaints and landlord-tenant disputes, the agency has a public utilities division that testifies before rate-setting boards on behalf of county residents. In addition, the agency licenses taxi drivers, vendors, gun dealers and gem dealers.