A key House subcommittee has approved administration proposals that would increase the level of health and domestic aid to El Salvador by nearly $20 million before Oct. 1.

However, the House appropriations subcommittee on foreign operations, chaired by Rep. Clarence D. Long (D-Md.), attached a variety of conditions that reflect congressional skepticism of the Salvadoran government's conduct of such non-military affairs.

The action clears the way for the Agency for International Development's "reprogramming" actions giving $3.4 million in computers and technical assistance to Salvadoran election officials and adding $16 million and redirecting $9 million in health grants for the strife-torn country.

A senior official in the State Department's Latin American Affairs bureau said yesterday that the administration has no objection to the committee's constraints. "The concerns expressed by the subcommittee are concerns we can accommodate," he said.

The conditions include that:

* Salvadoran elections, currently planned for next year, "be open to all parties, regardless of political philosophy"--a phrase that presumably includes the rebel forces in El Salvador. The rebels boycotted the March, 1982, elections and unsuccessfully tried to discourage voting.

* "Access to the media . . . be provided to all registered candidates and that procedures . . . be developed to ensure the secrecy of the ballot and an honest and full count of the vote."

* AID establish an independent oversight commission to "examine the overall health needs of the people of El Salvador and review and evaluate existing and proposed AID health programs."

* The commission "recommend measures to ensure . . . that medical and health care personnel not be subject to harassment, illegal arrest or disappearance."

One official familiar with the committee's stipulations said that they are partially the result of reports offered by a private group of scientific and medical personnel.

That group documented instances of harassment of medical workers and questioned the adequacy of medical training, particularly since the closure of the medical school of the University of San Salvador.