The Federal Home Loan Bank Board has proposed to liberalize its regulations on how large a down payment home buyers must make to qualify for a mortgage loan on a two-family house.

Bank board chairman Robert H. McKinney said the liberalized rules - which will permit buyers to put less than 20 per cent down - are needed "to encourage home ownership, principally in inner-city areas where low and moderate-income families may use the rental income from the second units to make their mortgage payments."

The Federal Home Loan Bank Board regulates federally chartered savings and loan associations.

At present home buyers can put less than 20 per cent down when buying a single-family house, but must put at least 20 per cent down when a multi-family house.

The bank board also proposed to increase the number of loans S&Ls may hold where the downpayment is less than 20 per cent.

A board memorandum said that a recent survey showed that "loans in excess of 80 per cent of value do not involve a higher degree of loss than that of loans of 80 per cent of value or less."

As a result, the memorandum concluded, restricting the number of these loans an S&L can hold on the "basis of excessive risk" is hard to rationalize.

When home buyers put less than 20 per cent down (that is, when S&L makes a loan in excess of 80 per cent of the value of the property) Most S&Ls require the home buyer to take out special mortgage insurance.

The bank board memorandum noted that increasing the percentage of assets a saving and loan may hold in these high value mortgages will increase the market for the private mortgage insurance companies.

However, "since many associations are quite far from our current asset limits," the memorandum said. "The expansion would most likely to be a gradual one, thus avoiding a windfall situation for the companies as well as a strain on their capacity."

The board memorandum suggested that many S&Ls are restricting their loans of more than 80 per cent of value in order to say under the bank board's ceilings.

A boost in the number of loans of this type that a savings and loan association can make "is needed to further ensure the adequate availability of funds for home ownership of low and moderate income groups," the memo concluded.

McKinney, a former Annapolis classmate of President Carter's, who was sworn in as chairman of the bank board last month, said in a statement that by permitting these low-downpayment loans on two-family dwellings, families that are at present renting because cannot afford to buy may be able to buy homes using the rent from the second apartments to in effect subsidize their mortgage payments.*