Washington area officials who are completing the federally ordered restudy of the Metro subway system charged yesterday that the federal government is changing the rules at the last moment, thereby creating more delay and added cost for Metro construction.

The issue, simmering since January, centers on the question of whether Metro can simply stop building at 64 miles of its planned 100-mile length. Adding weight to the question is the fact that it is being asked by several groups in the area who challenge whether Metro will be worth its escalating costs, and whether it will provide the transportation benefits Metro planners claim for it.

The 64 mile figure represents 60 miles of Metro that have received guaranteed funding and four more miles that have federal blessing but lack total funding.

If no more than 64 miles were completed, Metro would lose:

The Red Line from Silver Spring north to Glenmont.

The Orange Line from Ballston west to Vienna (or Tysons Corner, a possible option).

The Green-Yellow Line from Gallery Place north to Greenbelt.

The Green Line from Anacostia south to Branch Avenue.

The Yellow Line from King Street west to Franconia.

The federal Urban Mass Transportation Administration, in a letter from its chief, Richard S. Page, has asked the regional task force studying the system to provide "an incremental analysis by line of the proposed regional alternatives beyond the committed 64-mile system.

That is what members of the task force - political leaders from the Metropolitan Washington Council of Governments, the metro board and the regional Transportation Planning Board - think is a lastminute change of rules.

They were supposed to study alternate - but equivalent - public transit service for each of the corridors. "The Urban Mass Transportation Administration, in its infinite wisdom, concurred" in that definition of the study late in 1976, Metro board chairman Joseph Wholey said yesterday.

By asking for an "incremental analysis" for each line from 64 miles out, UMTA is "dreaming up new requirements," Wholey said. The study, Wholey and others explained, was never designed to answer questions about what would happen if nothing beyond 64 miles were built.

"We know the (dollar) cost of the extensions," task force chairman Francis B. Francois said, "but as soon as we accepted a truncation (of a planned Metro line), we bring up a whole other set of issues. Underlying them all is the political assumption that (such truncation) is not acceptable."

The 64-mile system would mean, for example, that downtown Silver Spring and the Parkington Shopping Center in Arlington - bot dense commercial areas - would have to take the additional traffic impacts of being the end of the line.

It would mean that there would be no line in Washington running north of Gallery Place into the 7th and 14th street NW corridors, and no line running through Southeast Washington beyond 14th Street and good Hope Road SE - just across the Anacostia River. It would mean that two rail yards needed to operate Metro would have to relocate somewhere else never considered, according to area planners.

The 81-mile proposed minimum system solve all these problems, and suggests alternative transit for the other areas.

The task force is about to present a public hearings extensive data on operating costs, ridership projections, etc, for the four proposed alternate systems. Ulimately, after consultation with UMTA, the Metro board and local officials will select one system and a financial plan to play for it. The deadline is August.

There is no money in the current proposed federal budget for further Metro construction, but Transportation Secretary Brock Adams has said that he would seek money after the study and the financial plan are completed.

The new request for "incremental analysis," if taken literally, would add weeks and months to the study, according to Wholey and others. No specific operating cost and ridership projections have been developed for a basic 64-mile system. For the record, the construction cost of 60 miles is currently listed by Metro at $3.8 billion.

William F. Moore, husband of Montgomery County Council member Jane Ann Moore, circulated a press release at a recent Metro board meeting asking that the "cost of a 64-mile Metro rail system be available" at the hearings.

Without such information, he charged, Metro "would be clearly misleading the public into thinking sufficient information for an intelligent choice has been made available, when it has not."

The regional task force voted yesterday to seek a meeting with WMTA's Page in an attempt to resolve the problem.

Transportation Secretary Adams, at a recent press conference, was asked if a recent series of federal letters to Metro was intended to impose more delays and require more study.

"No," he said. "I just want to get costs under control." After a pause, he said. "They cry a lot. Both sides." Asked later if that meant UMTA as well as Metro, Adams said, "Yeah."