Washington area federal commuters have a good shot at a $25 million refund -- it works out to an average of $340 per person -- of parking fees they have paid since late 1979. U.S. District Court Judge Harold H. Greene ruled last Friday that the Carter administration acted illegally when it ordered U.S. workers to pay for parking without consulting Congress.

Greene's decision, which invalidated pay parking at government offices effective March 13, sets the stage for an even bigger decision down the road when Greene rules on a motion by the American Federation of Government Employees. It wants workers paid back for all parking money they have caughed up since the program began. About 75,000 civil servants here had been paying from $12 to $40 per month to park in spaces they got free before the Carter order, which was a spinoff of his energy conservation program.

Uncle Sam is appealing the decision. But it will be sometime in September before the U.S. Court of Appeals issues its ruling. In the meantime, feds who had been buying daily or monthly parking stickers are exempt from pay parking. Some agencies have already stopped charging. Others, which collected monthly fees for March, will stop charging beginning April 1.

Federal officials say that even when free parking returns, parking spaces will be allocated on the same basis as now. For official spots that means mail trucks get first crack, then fire engines and law enforcement vehicles, the private vehicles of judges and members of Congress and then government owned vehicles.

For employe parking spaces, handicapped workers get first consideration for parking, then executives and personnel working unusual hours, then van pools (to be a van pool you must have eight or more, counting driver and riders), then car pools with those with the most riders getting first priority. A car pool must have a minimum of two persons (driver and passenger) and make a minimum two mile round trip.

So for now, pay parking is out. Probably until September. And there is a chance (although the government will certainly appeal an adverse ruling) the refunds may be ordered. Like we told you in the Feb. 12 column, hang on to your parking receipts and canceled checks. They could be worth some money in the future.