The government is cutting the interest rate on FHA and VA single-family mortgages by half a percentage point to 13 1/2 percent, effective today, the lowest it has been in the past year and a half.

It was the third drop since early August, when the FHA/VA rate was 15 1/2 percent.

Mark Riedy, executive vice president of the Mortgage Bankers Association, said mortgage rates probably are about at the bottom of their decline now, though he added they may stay down for months because of continued weakness in the economy generally.

The FHA/VA rate drop appropriately reflected recent changes in other mortgage interest rates, which have fallen to a range of about 14 1/2 to 15 percent, Riedy said.

Conventional rates, on mortgages not insured by FHA or VA, are set by individual lenders, generally according to market forces. FHA and VA mortgages are geared to first-time and lower-income homebuyers who cannot get conventional financing. The FHA and VA rates are set by the government and generally have been kept a peg or two below market rates.

HUD Secretary Samuel R. Pierce, in announcing the FHA rate decline, said it would enable thousands of families to buy homes.

Some housing economists have viewed 14 percent as a threshhold rate, below which they say people will start buying again.