The President's Commission on Housing endorsed yesterday the most massive structural changes in the nation's financial institutions since their basic framework was developed in the 1930s.

The commission's recommendations--some of which already have been proposed by the administration and introduced as legislation--involve sweeping deregulation of banks and savings and loans and would further blur the lines between the different types of institutions.

In addition, the panel would like to eliminate assumable mortgages, which are used in about 42 percent of all home sales being negotiated today, according to National Association of Realtors figures.

Assumable mortgages are about the only way a buyer can get a low-interest, fixed-rate mortgage today, and the real estate industry says their availability is the main reason it has been able to stay afloat at all in the current housing market slump.

But savings and loans have argued that the practice forces them to keep on their books a number of unprofitable older mortgages, contributing to the severe problems of their industry. The S&Ls have urged federal laws that would enable them to enforce "due-on-sale" clauses in mortgages that require that the balance of the loan be paid when a home is sold to another buyer.

The commission's due-on-sale recommendation was in keeping with its philosophy that the nation's mortgage lenders must be restored to health before the housing industry can be.

"Freedom on the part of the lender to cover himself would make it easier to provide fixed-rate mortgages. We want to create an environment in which a long-term, fixed-rate loan could become feasible," commission Chairman William McKenna said at a news conference to release the panel's preliminary report on long-term housing finance needs.

The commission will make a final report in April on all its findings to the president and Housing and Urban Development Secretary Samuel Pierce.

One recommended policy that the administration has not yet adopted would give banks the same tax advantages that S&Ls now enjoy for mortgage investment.

"The major flaw of the financing system has been a lack of flexibility that has rendered it fragile under pressure," the report states.

To change this, the commission would like thrift institutions to have more powers, including the ability to offer checking accounts to businesses and to make more types of consumer loans. At the same time, it would expand the ability of commercial banks to invest in residential mortgages and real estate.

The prevention of assumability and much of the deregulation policy envisioned by the report already is contained in a restructuring proposal bearing the names of Federal Home Loan Bank Board Chairman Richard T. Pratt and Senate Banking Committee Chairman Jake Garn (R-Utah) that will be the subject of hearings in the Senate in February.