The General Services Administration has concluded that a valuable 11-acre property on the Alexandria waterfront is "vastly underutilized" as a government warehouse and should be sold, possibly to the city.

Alexandria has been seeking to acquire the property, site of the old Ford Motor Co. facility at Franklin and Union Streets in Old Town, for inclusion in its rapidly redeveloping waterfront. But until now the government had been unwilling to part with it.

The site has been in government hands for 41 years, half the time as a Navy munitions plant and half as a warehouse shared by the Defense and Treasury departments and six other agencies.

Last fall, GSA administrator Gerald P. Carmen, following requests by Alexandria officials, ordered a study to determine if the public interest would be better served by keeping the property or by selling it to the city or a commercial developer.

B. Michael O'Hara, GSA's technical services director, said the study--now about to be released--concludes that the acre tract is "vastly underutilized."

The property, which includes deep-water berths, is an attractive site for mixed commercial and recreational development along the crowded and reflourishing Old Town waterfront. On Jan. 1, city tax assessors said the property was worth $15.8 million.

"With that kind of value, the current utilization does not appear to be optimum," O'Hara said. "That's an extremely high value for a warehouse facility."

Unless federal agencies now occupying the warehouse object strenuously enough to halt the disposal or unless another federal agency pays GSA $15.8 million to claim the property, negotiations will likely begin with the city of Alexandria.

Rep. Stan Parris (R-Va.) is urging GSA to give the site to the city. But the Reagan administration has decided to sell, rather than give away, unneeded federal properties whenever possible, regardless of local public benefits.

"For a little city like us, that's a pretty big price," said George S. Colyer, Alexandria's chief of comprehensive planning. "Maybe we could string out the payments to the federal government or provide some property in kind."

GSA's Public Buildings Commissioner Richard O. Haase said the federal government would consider those alternatives, "but we're not giving it away for nothing."

Parris said he will lobby the White House Property Review Board to make an exception for Alexandria.

"I will certainly attempt to convince them. . . ," Parris said. "The local uses transcend the interests of any federal agency that wants to use it as a storage place."

Parris said the tract is "absolutely critical to the balanced use of the waterfront."

Colyer said the city's money situation is tight because Alexandria has its "biggest capital improvement load in years" underway right now, "draining our reserves." The projects include a new jail, police headquarters, upgrading waterfront parks and renovating the Torpedo Factory.

"The most plausible solution is to have the controlled, proper balance of private development and recreational space," Colyer said. "But obviously this is going to be political because the Ford plant is in the heart of residential Old Town."

The Treasury Department's bureau of government financial operations runs a supply depot from the Ford plant and is GSA's major tenant. It pays GSA 32 percent of the rent it collects: $262,184 a year. The Defense Department operates a supply depot for the secretary of Defense and for the Air Force from the plant. In addition, the Army Corps of Engineers has a vehicle maintenance facility there. DOD pays $173,016 for its space.

Parris said he wasn't concerned about whether Treasury had any objections. Treasury Secretary "Don Regan is a friend of mine and a constituent," the congressman said. "He and I can have a heart-to-heart talk about this when the time comes."

It may be a little harder relocating the Defense contingent, which includes some landing craft docked on the Potomac River that may not fit well into a dry warehouse.

"Obviously, the boats are no good unless they are on the waterfront," said Jack Finberg, GSA's regional planning director. "It is decisions like that which are going to make the difference."

Finberg said he is writing a "regional opinion" to O'Hara's study and would not discuss it directly. One contention he is likely to raise is the cost of leasing warehouse space to accommodate the agencies that are now in the Ford plant.

"We have no space; we'll have to rent it," Finberg said.

But administrator Carmen said last week he wanted the National Capital Region to explore ways for the Ford plant occupants to be relocated to the old Washington Navy Yard warehouses that GSA operates. Those warehouses, primarily used to store furniture, have been the subject of intensive studies over the past month to determine how much space is actually being used and how much is vacant and available.

GSA regional officials, however, said they were skeptical about finding blocks of space large enough to accommodate Ford plant occupants. CAPTION: Picture 1, City tax assesors said in January that the old Ford plant at Franklin and Union streets is worth over $15 million. Picture 2, Depot manager Reginald Word checks inventory at the old Ford plant warehouse, which is now used by the Treasury and Defense departments. Photos By Douglas Chevalier-The Washington Post.