Montgomery County will begin offering $30.3 million for home mortgages to first-time home buyers today at 9.5 percent interest, one of the lowest rates since the program started six years ago, officials said yesterday.

"We hit the bond market at a fantastic time," said Timothy C. Mitter, manager of the mortgage finance programs at the county's Housing Opportunities Commission.

The mortgages are being financed by the sale of $35 million in tax-exempt bonds. After administrative charges and other costs are deducted, $30.3 million will be available for home mortgages, Mitter said.

The bulk of the money will be used to finance the purchase of 515 new homes in 18 new subdivisions around the county. For the first time since the program started in 1979, more than $3 million also has been set aside for the purchase of existing homes in more urbanized parts of the county, such as Silver Spring, Wheaton and Takoma Park.

In addition, $1 million has been earmarked to purchase existing town houses and condominiums that were built in the Gaithersburg-Germantown area under the county's Moderately Priced Dwelling Unit program, officials said.

All buyers must meet income limits that start at $26,000 a year for single persons and rise to $29,200 for couples, $32,000 for a family of three and $34,000 for a family of four, said Bernice Bass Palmer, coordinator of single-family programs.

The commission also placed ceilings on the price of homes that range from $55,000 for a one-bedroom condominium to $80,000 for a three-bedroom detached unit.

First-time buyers who are interested in buying a new house under the program should contact one of the participating developers or call 933-HOME for details, said Joyce Siegel, a commission spokeswoman.

Those who are interested in buying an existing house should find a unit that meets the program guidelines, obtain a sales contract and mail the contract and an application form to the commission. Application forms will be available July 1, Mitter said.

The participating developers are:

Greenhills Townhouse Condominiums, Damascus, 840-1545; Hamlet Station, Gaithersburg, 921-4101; Greenfields at Brandermill, Germantown, 428-4980; Rolling Spring Condominium, Silver Spring, 598-2110; Christopher Court, Gaithersburg, 948-0808; Greencastle Lakes, Silver Spring, 890-3535; Village on the Park, Gaithersburg, 921-0080; Waterbury Downs II, Germantown, 428-3531; Tiers of Silver Spring II, 622-1257; and several scattered sites in Kensington and Silver Spring, 384-7353.