Congressional plans for a new $70.5 million office building next to Union Station, to be used as administrative offices for the federal judiciary, scored a major advance this week.

The proposal moved through the House Public Works and Transportation Committee this week after clearing the panel's subcommittee on public buildings and grounds. It is now awaiting action by the full House.

As currently envisioned, the six-story, 460,000-square-foot building would be located east of Union Station, between Columbus Plaza and Second Street NE, with its back to F Street. It would be separated from Massachusetts Avenue by a landscaped park and parking area.

One use of the new facility would be to provide a permanent set of official offices for former Supreme Court justices. Currently, no such offices exist.

The property had been earmarked for private development just four years ago under legislation that gave the Union Station Redevelopment Corp. the right to develop it commercially if that would make the redevelopment of the old train station into a moneymaker.

But Elliott Carroll, executive assistant to the architect of the Capitol, said the law gave the Transportation secretary the option of refusing to allow the transfer and the Reagan administration has decided that it will exercise that option.

An Office of Management and Budget spokesman confirmed this week that the administration "quietly" told Transportation Secretary Elizabeth Hanford Dole on Feb. 15, 1983, that she should not "exercise the option to turn the property over to the private sector or to the redevelopment corporation."

The OMB spokesman, who asked not to be named, said that OMB Director David A. Stockman told Dole in a letter that the administration agreed with the redevelopment of the Union Station area, but that the private sector should not gain control of this valuable tract of government property.

There were now "no obstacles" to the proposed use of the site, Carroll said.

Keith Kelly, executive director of the redevelopment corporation, said he was "in sympathy" with the plans to develop the site in a way that is compatible both architecturally and "in spirit with what we are doing at Union Station. I would rather that the administration considered giving us the right to develop it ourselves."

Kelly said he had never seen the Stockman letter, but "had heard a lot about it."

He added that he believes the Union Station redevelopment is viable without the income that would have come from development of the nearby site.

The new building is supposed to be constructed, Carroll said, "with the same grand nature" as the front of Union Station and the adjacent Post Office. The congressional subcommittee, however, has cautioned White that the members would not tolerate expensive cost overruns and delays such as those associated with the construction of the Hart Senate Office Building.

Carroll said that the congressional construction projects are always done "prudently." For this project, he said, the "basic construction will cost about $50 million, with $5.5 million for feasibility studies, engineering and design work; $3 million to improve the plaza area and $2.6 million for construction management."

The construction issue came up because the General Services Administration had a prospectus pending before the subcommittee to consolidate four units of the federal judiciary into one unspecified leased location somewhere in downtown Washington.

Armed with the 2-year-old OMB decision not to allow the site to be used as part of the redevelopment effort, George M. White, the architect of the Capitol, worked with the administrative office of the courts to come up with the 460,000-square-foot building plan and submitted it to the committee.

The panel decided that if White could put the building up within five years, it would be more economical than GSA's leasing plan. GSA officials agreed.

Carroll said the project was previously outlined in the Master Plan for the U.S. Capitol, a document issued in 1981 -- though never formally adopted by Congress -- that contemplated the uses for the dozen blocks of property under the architect's control.

"The siting, and use of the building," he emphasized, "are no surprises."

But the size just might be a surprise.

James G. Whitlock, GSA's regional public buildings commissioner, said that the request from the administrative office of the courts was for only 173,000 square feet of space.

"They are making it as large as it is to accommodate future expansion," Whitlock said. The net square footage of office space that is likely to be in the new building on Capitol Hill was estimated at about 320,000 by Whitlock. Carroll, however, said that the usable space that the architect is planning will total only about 220,000 square feet because some space would be used for underground parking.

GSA's plan, submitted last September, would have eliminated judicial branch offices from the Lafayette Building (811 Vermont Ave. NW); 1100 17th St. NW; 1120 Vermont Ave. NW and 719 13th St. NW. The architect's plan includes moving a small unit of court employes from the Dolley Madison house at 1520 H St. NW and some of the warehouse space the courts now have in Forestville, Md.

Under GSA's plan, a total of 535 employes would be moved. The architect's plan would accommodate about 600 employes.

The legislation for the structure was proposed by a bipartisan group of members led by Rep. Robert A. Young (D-Mo.), chairman of the subcommittee. He was joined by the senior Republican on both the subcommittee and the full committee, and by the committee's chairman, James J. Howard (D-N.J.).

It could not immediately be determined whether there would be any opposition to the project from the Senate or the Senate Public Works and Transportation Committee.