Thousands of federal workers who now park near their Washington offices for free or at a nominal cost will have to start paying from $22.50 to $40 a month for the same parking spot Nov. 1 under President Carter's plan to promote energy conservation.

While the financial bite will be steep for many workers, they still will be paying only half of what the General Services Administration considers to be the going commercial rates at nearby privately owned lots.

But federal workers will feel the full impact of Carter's directive starging Oct. 1, 1981, when the GSA will charge the same rates for Federal lots as those imposed at nearby commercial lots, as much as $80 a month and possibly more.

Carter last April ordered the controversial new parking fees at government lots throughout the country as a means of promoting energy conservation, improving the nation's air quality and reducing traffic congestion.

But it was not until this week that GSA announced the rates that it will charge here and elsewhere. In the Washington area, the new charges will be imposed on about 55,000 parking spaces, with the government expecting to reap $17 million to $20 million in extra revenue each year. Nationally, the government expects to collect between $35 million and $40 million annually.

In the commuter-oriented Washington area, the government has predicted that the extra parking charges will help increase use of mass transit, eliminate 100,000 miles of daily car travel and save 6,000 gallons of gasoline.

The GSA rates only apply at federal agencies, not at the White House and on Capitol Hill. But GSA recommended rates for parking near the White House and some workers there probably will have the dubious distinction of paying the highest federal rate in the Washington area -- $40 a month -- for the next two years.

Q White House spokeswoman said officials are studying the GSA rate recommendations and likely will adopt at least the minimum rate suggestion. The $40 rate would apply for 270 parking spaces in the Executive Office Building. A government lot at 1800 G St. NW is the only other Washington lot that will charge as much.

Workers on Capitol Hill, who now park free, almost certainly will escape paying any parking fees for a while longer. A bill sponsored by Sens. Charles H. Percy (R-Ill.) and David F. Durenberger (R-Minn.) would impose parking charges on the Hill, but has yet to be marked up by a committee.

D. C. Mayor Marion Barry, Metro and the Metropolitan Washington Council of Governments have all supported imposing the parking charges on Capitol Hill and elsewhere, but the National Association of Government Employees and the American Federation of Government Employees have voiced opposition.

Under the GSA rate schedule, outdoor parking spots will cost $2.50 less a month for the next two years, $5 less thereafter, than nearby indoor parking spaces at the same agency. The plan calls for free parking for handicapped workers and vans carrying at least nine persons. Private cars carrying at least two persons will be assigned priority spaces, but the workers will still have to pay the full levy.

Workers in the main Interior and Energy Department buildings will have to shell out $35 a month for inside parking spaces, while those at the FBI headquarters will pay $32.50. A $30 monthly charge will be assessed at the main Labor, Justice, Housing and Urban Development, Transportation, Commerce and Health, Education and Welfare department buildings. The rate will be $27.50 at the Agriculture and State departments, $10 at the Pentagon.