Moderate-income residents of Montgomery County will begin moving next month into the first rent subsidy apartments funded by the county's three-year-old condominium conversion tax, County Executive Charles Gilchrist announced yesterday.

Forty-two units of the new Oak Mill apartments on Rte. 355 in Germantown have been reserved for qualified tenants, who will be charged 24 percent of their monthly income for rent. The county will pay the balance of the market-rate rents.

Gilchrist said the rental supplement "came out of our realization that many households were having trouble finding affordable housing in Montgomery County.

"Because of the loss of rental units to condominium conversion, especially between 1979 and 1982, there was a need to add to the stock of affordable rental apartments," he said.

Since 1978, about 7,000 apartment units have been converted to condominium ownership in the county, said Rick Ferrara, director of housing and community development.

The 4 percent condominium tax, implemented in 1981, is added when condominiums are sold for the first time. Yearly interest on $3.2 million of the $13 million in conversion taxes collected since the program began is being used for rental assistance.

Families displaced by condominium conversions are being given top priority for the units, although applications are still being accepted. Seven thousand families are on the county's waiting list for rental assistance. To qualify, applicants must have incomes ranging from a minimum of $12,000 for one person to a maximum of $23,270 for a four-person family.

Brenda Matthews, who is moving with her daughter into a two-bedroom apartment at Oak Mill in February, said, "there's no way I could make it" without the program. Matthews, 22, said she makes $600 a month with Watkins-Johnson Co., a Gaithersburg electronics manufacturer. The rent on her new apartment is $565. She will pay $144.

Tax-exempt financing that is 2 1/2 to 3 points lower than conventional financing is available to developers. In turn, they are required to maintain their apartments as rental units for 15 years.

Seventy additional rental assistance units will be ready for occupancy next year in two Silver Spring apartment complexes, officials said.