U.S. Postal officials are looking for 40 to 50 acres of land in Northeast Washington for a large new mail-handling facility and repair garage to replace the outmoded main city post office near Union Station.

The District of Columbia housing department wants the new facility located at Fort Lincoln, the city's "new town in town," because they believe it would provide an economic boost to the struggling project. But many Fort Lincoln residents oppose the facility, saying it would be an industrial intrusion that would dramatically change the character of the planned community.

Earlier this year postal officials talked with the city about acquiring government-owned land at either Fort Lincoln or Harmony Cemetery, adjacent to the Rhode Island Avenue subway station, because those sites met the agency's requirements of size and access to good transportation routes, said Robert Markham, the post office's manager of major facilities.

Last month, however, the Post Office decided to offer private developers a chance to bid on the project as well because, officials said, the agency thought such a request might produce a better deal.

The postal facility, which would be the largest in the Washington area and one of the largest in the nation, would occupy nearly 16 acres, employ 5,000 persons and operate 24 hours a day. The rest of the acreage would be paved to provide parking for 2,208 cars and postal vehicles.

No matter where the new facility is built, the post office facilities on the first floor of the old main post office at North Capitol Street and Massachusetts Avenue will remain open and the remainder of that building will be leased to other governmental agencies.

Postal officials have ruled that the new facility must be built in upper Northeast "because of ease of transportation to the freeway," said Markham.

There are two existing sites besides Fort Lincoln that meet the agency's land requirements. One is a 35-acre tract along New York Avenue, between Montana Avenue and Brentwood Road, owned by CSX, a large railroad conglomerate.

The second is on the north side of Brentwood Road, where four separate companies own a total of 33 acres.

The Government Printing Office had once planned to move to this site, which is bounded by Brentwood Road, Rhode Island Avenue and the T Street bridge over the northbound railroad tracks. The site is adjacent to Harmony Cemetery, and city officials said they would be prepared to provide a portion of the cemetery's land if it were needed to complete a postal facility there.

Fort Lincoln new town, a 360-acre city urban renewal project near Bladensburg Road and South Dakota Avenue NE, was envisioned at its inception in 1967 as a predominantly middle-class community of homes and apartments surrounding a town center with stores and office buildings and including its own schools, a college and a lake. The project has been plagued with problems.

Housing director Robert L. Moore said the new postal facility is needed "to spur and support further development," which has been stopped for nearly two years because of lack of housing sales, a common problem in today's housing industry. He said Fort Lincoln developer Theodore R. Hagans will bid on the project.

Last year the city government agreed to subsidize the purchase of nearly two dozen $70,000 town houses by moderate-income families after Hagans was unable to sell the homes on the private market. About 1100 families now live at Fort Lincoln and a majority receive federal housing subsidies.

Hagans said yesterday he was "considering" offering postal officials 35 acres in Fort Lincoln's still undeveloped 60-acre downtown. Representatives of CSX and the owners of the other site attended a meeting with postal officials this week to discuss the proposed facility.

"The community doesn't see any benefit that could be derived from this kind of usage," said Carrie Thornhill, president of the Fort Lincoln Civic Association. "Besides the large amount of space, we generally do not warm to the idea . . . because it is more industrial use that will have an adverse effect [including] serious traffic congestion and depreciation of property values."

While opposing the entire facility, Thornhill said the residents are most opposed to the maintenance garage. Postal officials say the garage must be built alongside the mail facility or no further than a mile away.

Another Fort Lincoln resident, Georgia Persons, a Howard University political science professor, said the real issue is whether "it is possible to maintain a viable black middle-class neighborhood without the encroachment of a whole lot of things that are undesirable. The vehicle maintenance garage is a real slap in the face."

Moore said he shares some of the citizens' concerns but added "the Fort Lincoln plan was idealistic and now we are in the real world of economics."

The city has asked the federal National Capital Planning Commission to consider changing the Fort Lincoln urban renewal plan to allow the postal facility in the community's downtown area. Action on the request has been delayed however, because NCPC officials said they need additonal information.