Sen. Robert T. Stafford (R-Vt.) is planning to take a long, hard look at a proposal by the architect of the Capitol to construct an expensive new office building next to Union Station for use by the Administrative Office of the U.S. Courts.

The $70.5 million construction project, which was rushed through the House Public Works and Transportation Committee earlier this month, is awaiting floor action by the full House.

But a senior aide to Stafford, who is chairman of the Senate Environment and Public Works Committee, said that the senator believes the project -- favored by Architect George M. White -- could be built for less if it were done privately.

The Union Station Redevelopment Corp. has an option on the property that could be exercised if the secretary of Transportation believes redevelopment of the train station couldn't move ahead without the financial incentives associated with construction of an adjacent office building that would be rented back to the government. However, the Office of Management and Budget instructed the Transportation Department early in 1983 not to exercise that option.

The Stafford aide said that Transportation Secretary Elizabeth Hanford Dole is taking a second look at the project, the OMB directive and the impact the legislation could have on the redevelopment.

Richard E. Burnette, DOT's senior spokesman, said he is getting "conflicting stories" from senior agency officials about what Dole is being advised to do.

"We've been fussing quite a bit this week to know just exactly what we will do," Burnette said. "I don't want to mislead you, so the best I can do is say at this point we don't know."

Harold H. Brayman, assistant staff director for the Public Works Committee, said the question is "who is going to be the master of this property."

"That property ought to be developed in conjunction with the station and not in conjunction with the whims of the architect of the Capitol," Brayman said, pointing out that the House bill moving swiftly to the floor just may have the counter effect of forcing Dole to decide if she wants to exercise the option to turn the construction rights over to the Union Station Redevelopment Corp.

Elliott Carroll, executive assistant to the architect, has said White regards the OMB letter as an inviolate decision that cleared the way for the bill to move forward.

In the past, White has been accused of engaging in a land grab on Capitol Hill, trying to expand the boundaries of the Capitol grounds as well as to put up new buildings. He has defended his efforts as prudent planning for the future needs of Congress.