The number of vacant rental apartments in the county is up slightly over last year, but the area still has a limited number of housing units, especially in the Bethesda-Chevy Chase area, the county Office of Landlord-Tenant Affairs reported.

The office said rents charged to new tenants this year have increased 5.5 percent over last year. As of April, 3.2 percent of all county rental apartments were vacant, up from 2.6 the previous June. The vacancy rate in Bethesda-Chevy Chase remained unchanged at 1.1 percent. Vacancy rates in other lower-county areas, including Silver Spring, Takoma Park, Wheaton and Rockville, all dropped below their 1984 levels.

However, those declines were offset by large increases in vacancy rates in the Gaithersburg-Germantown area and the Colesville/White Oak area, where there has been substantial construction in the past year, the office reported. Vacancy rates in those areas rose above 5 percent in 1985.

Average rents charged to new tenants countywide have increased 5.9 percent for one-bedroom apartments and 5.3 percent for two-bedroom apartments compared with 1984, the report said.

The average monthly rent for a one-bedroom apartment is $469, and for a two-bedroom apartment, $540, the office reported. About 38 percent of all units now rent for $450 to $550 a month.