Federal National Mortgage Association, a government-sponsored corporation criticized in recent years for not acting boldly enough to aid inner-city housing, plans to inaugurate three new programs within 90 days to help rehabilitate urban homes.

Oakley Hunter, chairman of Fannie Mae, as the firm is popularly known, said the new programs also would aid new construction in the nation's cities, But he said it is too early to know how much money might be involved.

Fannie Mae's new plans were revealed in a speech to the National Association of Home Builders in Las Vegas, at the same time President Carter submitted the administration's fiscal 1980 budget proposals to Congress -- calling for a decline in new subsidized housing units to 300,000 from 323,000 in the current year.

Presumably, a new effort by Fannie Mae to increase inner-city housing financing could halp to offset a decline in direct federal aid programs. However, the Fannie Mae programs are designed primarily to attract middleincome families back to cities and that could add up to more displacement for poorer residents.

As described by Hunter. the three new programs are as follows:

Financing of urban home acquisition and rehabilitation, under which FNMA would buy mortgages on existing properties in need of repair, with loans covering both the purchase (or refinance) and rehabilitation costs.

A mechanism through which commercial banks and savings and loan associations may borrow money from FNMA for urban reinvestment, with the financial institutions permitted to use the funds for commercial or residential revitalization pruposes.

Long-term commitments to cover lenders' urban loan mortgages, a "refinement" of a current program under which Fannie Mae commits up to 12 months in advance to buy urban mortgages.

"These programs offer a range of financing tools to the enterprising builder-rehabilitator who wants to break into the growing urban housing market (which) may be especially ripe now, as potential home buyers facing the alternative of soaring new home and transportation costs seek less expensive, more conveniently located older city homes," Hunter said in prepared remarks, which were circulated in Washington.

For the rehabilitation program, Hunter cited as an example a house now worth $20,000 which would have a $40,000 value after rehabilitation. Under the new plan a family could obtain a $38,000 mortgage loan from a FNMA-approved lender, making a $2,000 down payment. FNMA would immediately buy this loan and the $2,000 down payment plus $18,000 of loan proceeds would be used for acquisition and the balance placed in an interest-bearing escrow account, to be drawn on fr rebuilding.

Hunter said such funds would be provided at more favorable long term interest rates rather than short term (and higher) construction rates of today.

The new convertible urban commitments would not require the actual sale of mortgages but would assure a market for urban loans should the lender choose to sell them.

With a mortgage protfolio of some $40 million, Fannie Mae is the largest single source of home mortgage money. The firm supports a secondary market for mortgages, buying loans and thereby freeing up lenders' resources for additional mortgages.

Carter's budget message to Congress on Monday forecast that FNMA would make mortgage purchases or loans of $5 billion in 1979 and $7.2 billion in 1980 after the record $10.5 billion volume last fiscal year, when interest rates soared. Mortgage loan volume (receivables outstanding) are projected to increase to $41.6 billion this fiscal year and $44.7 billion in fiscal 1980.