While President Reagan was proposing a new department of trade last month, Congress was debating whether to reinstate the law that allows lower-level government reshuffling.

The legislation would renew a form of executive authority that expired in 1981. In the past, the law authorized the creation of such agencies as the Environmental Protection Agency, the Merit Systems Protection Board and the Federal Emergency Management Agency.

The House Government Operations Committee approved the reorganization bill Monday and sent it to the House floor.

The legislation would not give the president power to create new departments; for that he needs specific congressional approval. But it would allow him to create new agencies within existing departments or to transfer some federal functions from one agency to another.

For the first time, the legislation would require the president to give Congress written plans mapping out how the reorganization would be carried out. In testimony last month, Milton J. Socolar, special assistant to the comptroller general, said that earlier reorganizations had been hampered by poor planning and inadequate funding.

After studying six agencies that were created under the reauthorization authority in the late 1970s, Socolar said, "Start-up problems at the six new and reorganized agencies were severe. It took from 10 to 23 months to obtain key officials at two of the agencies. All six agencies experienced delays from nine to 30 months in acquiring other needed staff.

"Three of the reorganized agencies did not have sufficient funds . . . , and all six had difficulty obtaining adequate office space during the early stages of reorganization . . . . Obviously, much of the expected benefit of reorganization is needlessly lost or significantly delayed under these circumstances."

The new legislation has been endorsed by the Office of Management and Budget. Deputy Director Joseph R. Wright Jr. testified that the reorganization authority would not be used to tamper with the the Energy and Education departments. During the 1980 campaign, Reagan frequently pledged that these two creations of the Carter administration would be dismantled.

The bill would extend from 60 to 90 days the time Congress would have to consider a reorganization plan.