Home mortgage payments are not likely to change every month under the adjustable mortgage rate system that takes effect today, although they may fluctuate as often as twice a year or as seldom as once every five years.

"It would be a terrible job to adjust them [every month]," Frank L. Hewitt Jr., president of Citizens Savings and Loan Association, said yesterday.

That appears to be the consensus among area lenders grappling with the new regulation authorizing adjustable mortgage payments, which is regarded as a landmark change in the way Americans finance the buying of homes.

In the past, persons seeking to purchase a home typically were offered a fixed rate mortgage paid out over 30 years. The interest rate was fixed in advance and the payments were the same each month. In some cases, homeowners also paid their real estate taxes directly to the lender, and when taxes went up, so did the monthly payments.

Now, under the adjustable mortgage rule approved April 23 by the Federal Home Loan Bank Board, the federal savings and loan association can increase -- or decrease -- the interest rate and the payment in step with free market interest-rate changes. Faced with such uncertainty, consumers and others have expressed concern about the potential difficulty of financial planning when the monthly payments go up.

The rule applies only to new loans and not to existing mortgages. In any case, mortgage lenders say that at present there is little or no mortgage money available.

The thinking at Washington Federal Savings and Loan Association is that payments should be recalculated one or two times a year. "No less than every six months," said James L. Harris, president.

Customer payment schedules providing for identical monthly payments for periods of three to five years would be more appropriate, according to Hewitt of Citizens Savings and Loan Association. At the end of each three-to-five-year cycle, any additional money owned due to increased interest rates would be rolled into the principal and the customer's monthly payments adjusted for the next cycle, he said. If interest rates dropped during that period, the home buyer's payments would drop accordingly in the next cycle.

The adjustable mortgage loans can't be written for a period exceeding 40 years, according to the new regulation. That would allow a lender to spread loan payments over 30 years, with regular adjustments during those 30 years. Any money owned in additional interest then could be paid off during the last 10 years of the 40-year loan.

Although details of the new mortgage plans still are being hammered out in the boardrooms of the savings and loan associations, some area lenders already are tying the adjustable-rate provisions to new loans.

"We now have commitments with builders with these [adjustable] types of loans," said Kenneth M. Plant, president of Capital City Federal Savings and Loan Association.

Capital City stopped negotiating fixed rate mortgages last week, Plant said.

He said it was possible to make commitments on adjustable rates because of the 30 to 60 days that it normally takes for a loan to move from the application to the closing. By the time Capital City is ready to close the adjustable loans, the association will have done the paperwork necessary to comply with the regulation, Plant said.

Some other area lenders, however, don't expect to have adjustable rate mortgages available for weeks and perhaps even months.

"We don't have a firm game plan," said Robert A. Barton Jr., president of Interstate Federal Savings and Loan Association.

Barton also expressed concern about possible conflicts between some state laws and the new federal regulation. He said, for example, that some states forbid lenders from charging interest on interest. Yet, under the federal regulation, the unpaid interest that accrues on the loan may be added to the balance, and that sets up the possibility of the borrower paying interest on interest, Barton said.

A spokesman for the Federal Home Loan Bank Board said yesterday that the regulation is not subject to state restrictions.

Besides trying to shape mortgage programs that will comply with the law many lenders are waiting to see what rules will apply when they sell the adjustable mortgages to the Federal Home Loan Mortgage Corp. and to the Federal National Mortgage Association, which are known respectively as Freddie Mac and Fannie Mae. A savings and loan association has a choice of holding a home mortgage until it is paid off by the home buyer or reselling the mortgage to a second party. Freddie Mac and Fannie Mae will buy mortgages from the savings and loan associations and from other lenders, such as the national banks, if the original homeowner mortgages satisfy certain rules.

A spokesman for Freddie Mac said yesterday that a pilot plan for mortgage purchases will be announced in about 60 days. The plan will take into account the frequency of the home buyer's monthly payments, whether there is a cap on the amount of the interest adjustment that can be made during the life of the loan, and the index that is to used for making interest adjustments.

Fannie Mae also is developing purchase rules "as fast as we can," a spokesman said. No timetable has been set.

The importance of the Fannie Mae-Freddie Mac rules was underscored by Harris, the Washington Federal president. "A lot of the loans we will be making will be sold, and if Fannie Mae says to readjust the home buyer's payment monthly, every six months or annually -- that is what we would do. We would use the Fannie Mae model."

Otherwise, Harris said, the institution would be unable to sell the home mortgage and would have to hold it until the home buyer paid it off. Money that the savings and loan receives from the sale of the mortgage typically is lent to others seeking home mortgages.