THERE IS a long tradition in this country of encouraging the construction and sale of houses through preferential interest rates for mortgages. Those preferences are now rapidly disappearing. Recent statistics show a rise in sales. But it was apparently caused by very attractive financing, as builders desperately tried to lower their inventories. That's not a base for a continuing increase in home buying, any more than the automobile manufacturers' rebate campaigns are a base for an upward trend in automobile production.

Much more important for the year ahead, the Reagan administration apparently intends to reduce severely the operations of the Government National Mortgage Association and its role in the secondary market for mortgages. The administration has reportedly decided not to kill the GNMA, but it clearly foresees a much reduced federal role in housing. Shrinking GNMA's capacity to guarantee mortgage credit will, in turn, make it more difficult for lenders to offer FHA and VA loans, with their low down-payment requirements for people first entering the housing market.

Builders keep telling themselves that if the interest rates were only to come down a little more, their business would take off again with a roar. But would it? The accustomed levels of federal intervention and subsidy no longer exist. The whole savings and loan industry is in difficulty. There is no longer a large and profitable sector of the financial world devoted solely to mortgages. Lenders are already backing away from the conventional idea of a mortgage, at long term and fixed rate.

The structure of the American economy is being changed here, and it has heavy implications for the economic recovery that is supposed to begin around the middle of next year. In the past, the housing industry has always contributed powerfully to the recoveries from recessions. But when the current recession ends, it is very much an open question whether housing sales will begin to surge in the familiar manner--and whether the rest of the economy can recover without it.