The Reagan administration yesterday declared its opposition to the main Senate plan for aiding the housing industry this year, saying the proposed subsidies to lower mortgage interest rates would cost too much.

But the key Republican sponsors of the program plan to push the legislation anyway, hoping to get it to the president's desk next month. The measure, scheduled for markup in the Senate Banking Committee next week, was introduced by Sen. Richard Lugar (R-Ind.), Housing subcommittee chairman, and has the endorsement of Banking Chairman Jake Garn (R-Utah) and other Republicans.

The $5 billion, five-year program is designed to help an estimated 450,000 moderate-income persons buy newly built homes by having the federal government pay 4 percentage points of the mortgage interest rate. At the current FHA rate of 15 1/2 percent, this would mean the homebuyer would pay a rate of 11 1/2 percent for five years.

After five years, the rate would rise to the original FHA rate. When the home is sold or refinanced, the homeowner would have to pay back the subsidy immediately. The sponsors say all of the $1-billion-a-year initial cost will be recaptured eventually.

But Housing Secretary Samuel R. Pierce Jr. said yesterday that the administration could not support the mortgage subsidy plan because of its cost and impact on the budget now.

"It's a matter of money. The Lugar bill costs money, no matter what people say," Pierce said at a press conference yesterday. "We have to worry about this budget right now. We have to worry about this deficit right now."

Pierce called the press conference to announce administration housing initiatives that resulted from a Cabinet-level task force created by the president in February to find ways to help the housing industry.

The plans, which Pierce said have been approved by the president, consist mainly of regulatory changes that already were in the works before the task force was formed and would cost very little.

Representatives of the housing industry expressed disappointment that this was all that emerged from the task force, though they support the modest measures that were announced.

"It's like putting cosmetics on a corpse," said Mark Riedy, executive vice president of the Mortgage Bankers Association. "It doesn't do anything. . . . To call it a housing stimulus is a sham to the public."

The announced measures include a small increase in subsidies to about 70,000 units of low-income housing that have been approved for construction but have not been built because of high interest rates, FHA backing for new types of mortgage instruments, and rules changes designed to get more money from pension funds and private investors into mortgages.

Pierce also said that his department would end technical restraints on conversions of rental apartments to condominiums.

The secretary estimated that the actions would stimulate construction of 120,000 to 200,000 homes that otherwise wouldn't be built, but housing industry experts doubted the effect would be anywhere near that great. The industry expects about 1 million housing starts this year, about half what they were four years ago.

Senate Democrats also have endorsed the mortgage interest subsidy idea, although their proposal, unlike the Republicans', would apply the program in part to resales of homes and to homes that already have been built but remain unsold.

Lugar and Garn apparently succeeded in keeping their bill free of Democratic amendments, at least in committee. The two had canceled a scheduled markup earlier this month when it appeared the amendments would be approved. They now have the votes to get the bill out of committee without amendments, aides said.