Robert Brandt, a 24-year-old Washington land developer who calls himself "the newcomer on the block," successfully bid $950,000 this week at a federal property auction for less than a third of an acre of land in the 4300 block of Massachusetts Ave. NW.

Several residents in the neighborhood say they are upset because the federal government sold the undeveloped land and fear that further development will make the already congested upper Northwest Washington neighborhood worse.

But Brandt, who said he has not "even set foot on the property," said he has not decided what to build at the site, a small, triangular-shaped lot. Given the residential zoning on the block, Brandt said his choice most likely will be some sort of residential construction, possibly town houses or a condominium building.

"But I won't rule out anything," said Brandt, who added that, within six months, "You'll see bulldozers there, I hope."

Brandt's offer, at $67 a square foot, is considered high for a speculative, residential land sale, especially because of the small size of the lot and the limitations on what can be built there.

The property, 0.32 of an acre, just south of Ward Circle, was sold as surplus land by the General Services Administration at an auction that attracted 10 bidders. Brandt did not enter the competition until bidding reached $600,000. The lot was sold in less than five minutes.

The land, part of the Naval Security Station, was sold by GSA in keeping with an executive order issued last year by President Reagan that requires government agencies to either put surplus land to use or or to sell it.

Rufus Lusk III, of Rufus S. Lusk & Son, a real estate sales reporting service here, said that whatever Brandt builds at the site, he will most likely have to charge hefty prices if he is to recoup the land-purchase price.

"It's a very steep price and I guess they obviously see some value in the place," Lusk said.

Before any construction could begin, the unzoned land would have to be put into a specific zoning category by the District government.

Cecil Tucker, the acting chief of the District's zoning commission, said that, given the surrounding area, the lot most likely would be zoned to accommodate low-rise residential units, such as a garden apartment building or town houses, or a mid-rise condominium building that could not exceed 60 feet in height. A number of other nonresidential uses could be permitted, such as a health clinic, church, youth home, private club or museum, Tucker said.

But some neighborhood residents oppose development of the site.

"We're fighting it tooth and nail," said David B. Richardson, a board member at the Greenbriar condominium building, which is immediately south of the lot. Richardson said his group opposes use of the site for a chancery, an office of nearby American University or a residential complex -- all of which he said are possibilities.

He said parking is a problem for Greenbriar owners, which would be exacerbated by any development.

Richardson said that at one time several Greenbriar owners were interested in buying the property to keep it as undeveloped land, but admitted their possible offer wasn't close to the $950,000 Brandt is paying.

Carlyn Robinson, a Greenbriar resident for 13 years, said the peaceful, tree-filled view from her balcony and bedroom will be marred by the prospective construction.

"We're very worried what's going to go here," she said. "In Washington, if you've got a piece of green, they've got to put concrete on it. If it gets too bad, I'll sell my apartment and move."

Erling Hansen, chairman of the area's Advisory Neighborhood Commission, said his group is concerned about the precedent that the sale sets because of the vast federal land holdings in Ward 3, the area of the city west of Rock Creek Park and part of Georgetown.

William Holcomb, a GSA realty specialist, denied there is any overall trend toward selling federal land in the Washington area, but acknowledged the agency is studying a number of local properties to determine if they are surplus federal land. Holcomb declined to identify the sites.

Hansen said the so-called "privitization of federal lands" through such sales as the Massachusetts Avenue auction, while not having "any assurance that the zoning will match" an existing neighborhood, has upset some area residents.

"All I can picture is a narrow, Empire State type of building built there to recoup the $950,000," Hansen said.

But Keith Kelly, president of the residents association at the Westover Place town house complex, across Massachusetts Avenue from the lot, said, "I suspect as long as it's residential we have no problems with it." He said his group was more concerned about commercial congestion in the immediate vicinity.

Brandt, who has 60 days to complete the purchase, said the prime Massachusetts Avenue location convinced him to buy.

"It's a sellers' market now, and I just wanted to get a piece of it," said Brandt, whose only other construction deal was the recent development of 17 town houses on upper Connecticut Avenue.

Brandt's father, Lawrence, is a major Washington-area land developer, whose projects include the Sutton Place residential and commercial complex a few hundred yards southwest of the lot the younger Brandt bought.

"This area has been good to him and I hope it'll be good to me," the younger Brandt said.