A Montgomery County housing agency sold more than $28 million worth of bonds yesterday in order to offer mortgages at 8 7/8 percent -- far below the prevailing rate -- to 460 home buyers in the county.

The mortgage money will become available July 15, but only families with moderate incomes will be eligible to apply. For example, a family of five would qualify for a 30-year mortgage to buy a $55,000 home if its income were less than $31,600 a year.

The mortgages to be offered by the County Housing Opportunity Commission may be applied to housing in the $50,000 to $65,000 range; the 8 7/8 percent interest rate could save home buyers as much as $250 a month in mortgage payments over conventional mortgages, which are two to three percentage points higher.

About 40 percent of the annual demand for 7,700 housing units in Montgomery County -- where the average price of a single-family home is $110,000 -- if for moderately priced units.

Although the bond sale will finance only 460 mortgages, the news cheered developers who have been hard hit by slumping housing sales.

"It's a lot of help to us," said William Hussman of Kettler Brothers, which has already started construction on 62 homes in Montgomery Village that will be available to buyers with mortgages backed by the housing agency. Those homes, he said, "by and large . . . will be homes that wouldn't be built" without the special mortgage program.

Last the HOC sold more than $57 million of tax-exempt bonds and used the proceeds to float 1,100 mortgages at 7 3/4 percent for moderate income families.

The rate the agency offers is "a good rate for the handful of people who will get them," said Alan Wade of the U.S. League of Savings Associations. "But only a handful of people will be helped. Everybody else is left out in the soup."

The bonds fetched a low interest rate because they are not taxed by Maryland or the U.S. government, and thus are attractive to investors. "The more attractive a bond issue is to an investor, the lower the interest rate you can sell it for," said Joyce Seigal of the county agency. "If it's a real low interest rate, we can offer a real low mortgage. This is the lowest around."

Twelve developers in 23 subdivisions from Gaithersburg and Germantown to Potomac have houses already finished or under constructiuon that qualify for the low mortgage rates.

Prospective home buyers who meet the income requirements are sent to conventional lenders by developers and those lenders make the loans under the special program.