The Arlington County Board decided last night to explore the possibility of asking landlords to voluntarily agree not to convert their buildings to condominums in return for reduction of their county tax assessments.

The board asked its staff to research the legality of so-called "voluntary incumbrances" -- agreements entered into by the landlord and the county -- that would be taken into account when the market value of apartment buildings is determined by the county assessor.

Assessments on garden apartments in Arlington soared by as much as 47 percent this year, causing concern that the county was encouraging conversions to condominiums despite policies aimed at protecting housing for moderate-income families.

The higher assessments were prompted by a recent rash of apartment building sales to condominium converters. According to County Assessor James Vinson, the high sales prices paid redefined the market value of apartment buildings, regardless of whether they are still being rented.

Vinson last night vigorously defended the county's assessment practices, which he said were dictated by Virginia Supreme Court rulings on the definition of "highest and best use," the standard set for appraisals by the state constitution. "Fair market value is fair market value," he said.

"Since I became county assessor in 1977, I've heard complaints that multiple family dwellings were not bearing their full burden," he said. "Now, for once, we have an active market for apartment owners to base market value on."

Board member Dorothy Grotos took exception to any solution for apartment owners that would hurt homeowners. "Over a number of years single-family homeowners have seen their assessments increase while the apartments stayed the same. Now the situation is reversed and I don't want to get carried away here and push it back on the single-family homeowner."

Board member John Milliken proposed voluntary incumbrances as the "more modest" of various suggested solutions.

By agreeing to an incumbrance such as the one envisioned, a landlord would state that he does not intend to convert to condominiums. The board indicated last night it would seek back taxes from apartment owners who reneged on incumbrances and went ahead with conversion.

Milliken said a reclassification of rental property, in the mold of Virginia's farmland preservation program, would require a change in the state constitution. And, he said, another alternative that would allow apartment owners to qualify for tax breaks designed for urban homesteaders would be too unwieldy.