The average monthly cost to rent an apartment or town house in Fairfax County last year was $615, almost double the Washington area average and 8.7 percent more than in 1985, according to a new survey of the county's rental housing market.

For the second consecutive year, the number of rental units in the county increased, from 39,236 as of January 1986 to 40,003 as of this January. At the same time, the rental vacancy rate went from 2.5 percent, or 986 units, to 4.4 percent, or 1,773 units, the report states.

As property values in the county have skyrocketed in recent years, developers have been less inclined to build apartment complexes, which return smaller profits than commercial, industrial or mixed-use developments. At the same time, many apartment complexes have been converted to condominiums.

As a result, while the population of the county grew from 537,200 in 1975 to 683,800 in 1986, the number of vacant rental units dropped during the same period from 1,892 to 1,773.

The survey, which is conducted annually by the county's Office of Research and Statistics, reviews the rental housing market in all major rental apartment complexes and rental town house projects in the county. The survey does not include rental projects with fewer than five units and housing units that are leased to renters by individual owners.

According to the report, there were 982 newly available rental units and a loss of 156 units to condominium conversion, for a net increase of 726 units. The Springfield District, with 459 units, posted the largest increase (75.2 percent), while the Annandale District lost 413 units (19.2 percent).

Annandale posted the largest increase in the average monthly rent, 14.9 percent, climbing from $629 in 1985 to $723 last year. Rents were lowest last year in the Lee District, where the average unit went for $550 a month.