The Federal Housing Administration's insuring authority expires next Wednesday, but Congress is expected to extend the agency's authorization before the deadline, spokesmen for the House and Senate housing subcommittees said this week.

The FHA's power to insure home loans expired six times last year, requiring Congress to approve new legislation extending the lending authority.

On several occasions, bills were passed in time to prevent a break in FHA operations, but in other cases the insuring power lapsed, extending already long delays in loan processing. The shutdowns came in the midst of an avalanche of home buying and refinancing touched off by low interest rates.

Bills to extend the insuring authority for one month beyond its Wednesday expiration date have been introduced in both houses of Congress and the final legislation is expected to pass in time to avoid an interruption in mortgage loan processing.

A longer extension was not proposed because "we're on the verge of wrapping up a housing authorization bill, but we can't do it in the next 10 or 12 days," said Gerald R. McMurray, staff director for the House Housing Subcommittee.

He said the Senate and House conference committees expect to finish work on the bill and have it ready for consideration by the full Congress in early October.

The conference committee already has agreed on a provision to give the FHA permanent insuring authority, eliminating the need for periodic congressional action, according to a staff member.

Because Congress is certain to approve any legislation authorizing the popular mortgage insurance program and President Reagan is certain to sign it, less popular bills frequently are attached to such measures.

The traditional tussles were intense last year as members of Congress debated controversial housing legislation, and real estate industry participants said they fear a repeat performance this year.

"We expect once again that FHA authority will be a political football, and the home buyer gets used," said Warren Lasko, executive vice president of the Mortgage Bankers Association of America.

Unless Congress makes substantial changes in the housing bill being considered by a conference committee, Reagan may veto it as too costly.

Although Congress might try to override the veto, the delay could lead to a second lapse of the insurance authority.

About 60 percent of FHA home buyers are buying their first home, and their average annual income is $37,000.

The program, created in 1934 to help moderate-income Americans buy homes, has issued nearly $460 billion worth of mortgage insurance.

Although there is no limit on borrowers' incomes, the loan amount that can be insured is capped at $67,500 except in high-cost areas where the maximum can range up to $90,000.

FHA officials have approved $76 billion worth of insurance applications so far this year, well under the $100 billion limit set by Congress.

The bill to extend the FHA's insuring power also would extend for 30 days the Farmers Home Administration loan authority and the Home Mortgage Disclosure Act, which requires lenders to disclose information on lending practices in low-income city neighborhoods.