Like more than one local entrepreneur, the federal government is looking at the spots where cars are parked in Washington and seeing green.

In the latest twist to the Reagan administration's effort to make better use of federal property, the General Services Administration has begun contracting out government-owned parking lots to private firms.

Contractors now pay the GSA about $855,400 a year for the right to operate six parking lots, in some cases only after working hours and on weekends and holidays. Three of the lots have been turned over in the past 12 months.

Ellen Dayton, acting chief of the GSA's regional utilization branch, in the Washington region, is looking at other large lots near the Mall and other tourist areas that may be suitable to turn over to contractors.

"It's an incredibly lucrative venture," Dayton said. "Part of the reason for establishing an 'outleasing' program is to generate more revenue for the federal government and make better use of the space."

The GSA manages most of the federal parking spaces in the Washington area -- 56,205 to be exact, most close to the Mall or at the Pentagon.

In the past, only the huge, 1,810-space lot at the Great Plaza, behind the District Building, was run by private contractors for the GSA. But in the 1970s, the GSA contracted out two other lots: one at 12th and C streets SW and the other at the Pentagon.

About that time, President Jimmy Carter tried to get federal employes to pay at least 50 percent of the value of their parking places, in part to encourage them to use mass transit. But the federal employe unions resisted, and a court ruling barred the GSA from collecting those fees.

Some employes now pay their agencies small fees designed to cover the cost of managing the lots, while handicapped people and car-poolers who get first crack at unassigned places in the Great Plaza and 12th Street lots pay $15 a month.

Presumably, lots could be turned over to contractors who would then charge agencies or employes more to park there. But Dayton says the GSA is not interested in getting employes to pay the fair market value for their spots. "We're not aiming at making money from the employes."

By law, 88 percent of the revenues from the parking contracts go to the Land and Water Conservation Fund to help buy new national park land, The rest goes to a special treasury department account to offset costs of disposing of surplus federal land and property.

Frederick L. Sims submitted the winning bids on two of the lots that the GSA has turned over to contractors.

"I'm speaking from the perspective of someone who lacks the financial wherewithal to acquire land in major cities to run parking lots," Sims said. "The government is opening up major new opportunities for me and other small businessmen."

The lot that Sims runs at 1st and E streets has been particularly lucrative. When the Reagan-Bush campaign committee moved in next door, Simms raised his monthly rate from $57 to $69 and his hourly rate from $1.50 to $2.25.

"They paid and I made a bundle," Sims said. He said he plans to lower his monthly rate at the end of this month, when committee employes' contracts expire, but keep the hourly rate. The GSA does not regulate what the private vendors charge.

"It's what the market can bear," Sims said. When he took over 66 spaces at the USDA, he charged $2.25 for all-day parking on weekends and found that he could fill his lot. The next week he raised the rate to $5 and nearly filled it. The GSA charges him only $6,000 a year to rent the spaces.

William B. Jenkins, GSA's regional real estate chief, said, "It just didn't make sense to continue to leave parking spaces closed off in the evenings and on weekends, where there were people who wanted to use them and to run them."

"In many locations, there's not going to be any interest in running them in the off hours -- who would want to go there?" Dayton said. "We're going to be looking primarily at the lots near the Mall."

Another businessman who has profited from the new policy is Alex Gamble, who owns Colonial Parking and now runs the lot behind the Old Post Office Building after regular office hours.

"Our biggest problem is that no one knows we're there," Gamble said. "But it's an opportunity we would not have had without GSA.