The Federal National Mortgage Association is planning a special security that will allow banks and savings institutions to reinvest profitably in housing any deposits they attract under the new All Savers certificate plan that takes effect Oct. 1.

One effect could be to curtail the amount of new mortgage money that some proponents of the savings certificate had expected. Banks and S&Ls can loan money to FNMA for one year, in effect, and won't be forced to make long-term mortgage loans at bargain prices just to comply with a requirement that most of the new investments go to housing.

And Federal National Mortgage could substitute the new security for more costly Wall Street offerings -- all without boosting the total amount of money going to mortgages.

David Maxwell, chairman of the Washington corporation that is known as Fannie Mae, told a meeting of mutual savings bank officials that details on the debt instrument will be announced later this month. But Fannie Mae's new one-year security will have a higher return for the financial institutions than they have to pay savers.

The recently enacted tax law authorizes a tax-exempt savings certificate and requires that 75 percent of the funds or net new savings generated by this certificate be invested in housing or agricultural loans. Fannie Mae securities are mentioned specifically in the law as qualified residential lending.

While Maxwell was vague about details of the Fannie Mae program, he said it would provide banks and savings and loans "with a simple way to comply with the letter and spirit of the law, while locking in a favorable positive spread the difference between costs and return on certificates over the cost of new one-year All Savers deposits."

For consumers, interest on the certificates will be equivalent to 70 percent of the investment yield on one-year Treasury bills.