WHEN YOU GO into a savings and loan association to borrow money to buy a house, you are told that you must pay, say, $200 a month for the next 30 years - unless you live in California. There, you may be told it will cost you $200 a month now and may cost you, say, $220 or $200 or $180 five years from now depending on how the interest rate changes. That's one form of what is known as an "alternative mortgage." There are others, and the chairman of the Federal Home Loan Bank Board, Robert H. McKinney, has asked Congress to let federally chartered savings and loan associations offer three kinds of them. It is a proposal that deserves careful consideration. The traditional mortgage has worked well for generations of home owners. But the loan system needs more flexibility now to accomodate the pressures of a steady inflation and the changing needs of families.

The main impetus behind Mr. McKinney's proposal is the eagerness of the S&Ls to make loans, like those in California, that carry variable interest rates. With a mortgage of this type, your monthly payments change as interest rates in general change. There is a certain fairness about this. You pay the going rate, not 9 percent when rates drop (if they ever do) to 6 percent or 6 percent when they go up to 9. And, in theory at least, the original interest rate on mortgages of this type would be somewhat lower than on flat-rate mortgages. This is because the S&Ls would not have to worry about being stuck over a long period of time, as they are now, with mortgages on which the interest rates is below the current market. But there are also disadvantages. One is the possibly that monthly payments might grow beyond a borrower's ability to pay; this might be eased by lengthening the period of the loan. Another is the danger that, by tying the mortgages to the general interest rate, one of the incentives to keep the rate low would disappear.

The other two kinds of new mortgages proposed by Mr. McKinney are even more attractive. One of these would authorize lower payments during the early years and higher ones later on. That would permit some young people who can reasonably anticipate higher earnings in few years to buy a house sooner than they now can. The other would almost be a mortgage in reverse - an arrangement through which elder persons could borrow for living expenses against the value of their homes. Many people are now forced to sell because they can't make ends meet without spending part of the capital tied up in their homes.

The key consideration in all of these proposals is flexibility. Current regulations do not give the S&Ls enough. And what little there is seldom works to the advantage of those who would buy houses. Too much of the mortgage business involves fitting people into established categories that often have little relationship to individual situations. In opening up the field of mortgages for S&Ls, Congress ought to make sure that some of the maneuvering room is given to the borrowers. It should also consider barring the practice of charging "points" on home mortgages. This pernicious device was developed precisely because of the lack of flexibility - and the S&Ls should be willing to give it up in exchange for all or part of what they are asking.