The hard-pressed housing industry believes that Congress has thrown it a life-saver, in the form of the much-heralded all-savers certificate provision in the recently passed tax bill. But others say the industry may merely be clinging to as leaky raft.

The National Association of Home Builders and the National Association of Realtors predict that sale of the tax-exempt certificates -- one-shot deals that last 15 months -- will help to finance the construction of 400,000 to 450,000 new houses during fiscal 1982 and 1983. That would raise construction 40 percent over current levels.

The theory is that, with subsidized interest rates, more money will flow into S&Ls, which then can use it for home loans. At the other end of the transaction, the lenders are expected to be able to charge somewhat lower mortgage rates, and more people then would be able to qualify for home loans.

Because the certificates will be sold at 70 percent of the current interest rate on one-year Treasury secruties, lending institutions will be able to offer mortgages at rates from 2 1/2 to 3 percentage points lower than current ones, the U.S. League of Savings Associations estimates.

Jack Carlson, chief economist for the National Association of Realtors, predicts that an additional 5 million households will be able to qualify for loans at the reduced rates. Half a million people will be able to afford better residences, he also says.

These projections are considerably more optimistic than those originally made by congressional experts. The U.S. League has estimated that the tax-exempt certificates would increase deposits in the first full year by $20 to $30 billion, but the Joint Committee on Taxation put the amount at about half that.

The law contains sufficient flexibility to raise questions about how much it will do for housing. "The All-Savers Act is primarily intended to bail out S&Ls, secondly to add some net savings, and -- only finally -- to assist housing," one housing specialist said.

The law requires commercial banks and savings associations to invest 75 percent of all net new funds -- the amount deposited in All-Saver certificates minus the amount withdrawn from other accounts -- in residential housing or agricultural loans.

But the law also gives plenty of leeway in its requirement, allowing a financial institution to invest in conventional or government-insured mortggages, home improvement loans rehabilitation, cooperative apartments and mobile homes as well as securites issued by secondary-market agents such as the Federal National Mortgage Association. It does not require a lending institution to increase the proportion of mortgages in its portfolio.

So an S&L might be prompted to create new mortgages with the 75 percent new money, turn around and sell them in the secondary market and use the cash to improve its profit picture rather than making more new mortgage loans.

And despite the predictions of significantly lower mortgage rates, one skeptical housing economist believes there is a greater likelihood that rates will come down only one point and that thrifts will apply the other 2 percentage points to boost their earnings.

Federal Home Loan Bank Board Chairman Richard Pratt also expresses skeptism about the effect the tax changes will have -- or should have -- on housing. Pratt has said that the real bonanza of the All-Savers certificate is in maintaining the health of savings and loans so they will be able to resume mortgage lending when interest rates come down. He contends that if Congress wants to create a housing policy, it should do so, rather than trying the indirect approach of tinkering with the tax code.

The tax bill also contains several other provisions that will have a substantial impact on housing.

The greatest of these is the revision of depreciation rules that will shorten the economic life of residential buildings and increase the incentives for investors to build them.

NAR's Carlson predicts that within three years the leverage effect of the depreciation changes will result in a 5 to 10 percent additonal investment in new rental apartments.

Another consequence of the new depreciation rules is that builders of residential property will be able to compete on an equal basis with non-residential builders for funds. The recapture of depreciation on real property was made slightly more advantageous for the residential bulder than for the commercial builder.

Other tax provisions benefit the homeowner. On revises rules on the once-in-a-lifetime tax-free sale of a principal residence by a person over 55, raising the tax-free amount from $100,000 to $125,000 in gaings.

The amount of time a person can hold the proceeds of a house sale or exchange tax-free, before putting it toward buying another home, has been increased from 18 months to two years, effective July 20, 1981.

Tax credits for rehabilitation of properties have been liberalized to give a substantially beter incentive for fixing up old buildings in general and certified historic places in particular. The law replaces a variety of inducements with a three-tier investment credit, ranging from 15 to 25 percent.