General Services Administrator Gerald P. Carmen has urged the White House to order all federal agencies to reduce their office usage by 20 percent over the next five years--by giving up some rented space and consolidating in government-owned buildings. GSA officials estimate that the initiative would save more than $800 million by 1987.

Nationwide, the government currently is housed in 140 million square feet of office space, half of it leased. In the Washington area, it rents 23 million square feet and owns 22 million square feet.

Carmen is proposing to reduce the average space alloted each federal worker from 167 square feet to 135 square feet, bringing it closer to the nationwide average for commerical office employes--between 100 and 128 square feet, according to the GSA.

"We've found that most people can operate with 120 square feet per person," asserted James G. Whitlock, assistant commissioner for space management.

Although it was delivered late last year, the proposal "just became a hot item last week," Whitlock said.

"It's cooking, but it hasn't come to a boil yet," an Office of Management and Budget official said of the proposal. No decision has been made yet, but a preliminary conclusion is that federal agencies do appear to use more space per employe than the private sector, he said. "This one is being looked at intensively."

Some GSA officials say they are expecting an executive order in the near future, but the OMB official said an executive order is just one possibility. It also could be included as part of the budget proposal, he added.

GSA officials say the potential saving has political merits that make it attractive for the administration because, as one said, "We're not firing anyone, and we're not taking away anyone's food stamps." Whitlock acknowleged, however, that the proposal would be difficult to administer. "I will not dispute the dollar savings, but I know that to meet the time frame would require a tremendous amount of effort," he said. He also speculated that the proposal will encounter resistance from federal employe unions.

Carmen, who was named to the government's housekeeping agency seven months ago, is practicing what he preaches. He recently ordered a consolidation at GSA headquarters at 18th and F streets NW by pushing together desks, for example, and putting two persons into offices that formerly held one.

To fill the resulting 33,000 vacant square feet, Carmen last week moved in 220 employes from GSA's Federal Property Resources Service who used to work in 41,000 square feet of leased office space in Crystal City. GSA estimates that the resulting 20 percent reduction in office usage will save about $250,000 September. This figure does take into account the $25,000 it cost to move the employes and supplies. No other nationwide estimates for moving costs were available from the GSA.

Most of that saving will accrue from the reduction in rent from the $18.25 per square foot paid at the privately owned building to the $8 per square foot "rent" that federal agencies currently pay into the the Federal Buildings Fund used for repair, renovation and construction. However, one GSA official pointed out that government building "rental" fees will go up to $18 a square foot on Oct. 1, eliminating the lease differential as a significant source of savings.

Other savings will result from consolidation of building security and maintenance services, GSA spokesmen said. As an example, Carmen also is initiating cost-saving programs at GSA headquarters for items such as telephone equipment and office furniture.

The concept of consolidating office space to save money met even more skepticism from Robert A. Peck, an aide to the Senate Environment and Public Works Committee, which oversees the GSA.

"It's a real difficult thing to play with the number of square feet per person," Peck said. "It would be difficult to enforce."

Peck predicted that the difficulty of squeezing people together would hamper the consolidation effort. "Does everyone think the assistant supervisor with a private office is going to move someone else into his office?" he asked.