Fairfax County homeowners face a greater risk this year of having their property overassessed. Plagued by economic recession, an anemic real estate market and cutbacks in state aid -- and committed not to raise the real estate tax rate this year -- the county is desperate for revenue. Under these circumstances, taxpayers need to be wary lest the county be tempted to cushion its revenue shortfall by squeezing 1991 home assessments above current market prices.

There is ample reason for concern. A report last May by Fairfax County's Board of Equalization of Real Estate Assessments disclosed that there has been frequent overassessment of commercial real estate in the county. Our own more recent analysis of residential properties indicates that the county also overassessed a large number of single-family homes last year.

We compared the sale prices and 1990 assessments for the nearly 400 existing single-family detached homes in Fairfax County listed by the REDI Real Estate Information Service as having sold in January 1990. We chose January since it is the same month as the county's assessments, so there is no possibility that differences between prices and assessments could be caused by interim changes in market conditions.

The comparison showed that one out of six homes sold at less than its assessed value. These homes (houses plus lots) were in every area of the county and all price ranges. In half the cases, the assessments exceeded sale prices by more than 5 percent. In a third of the cases, assessments exceeded selling price by more than 10 percent.

These findings cannot be attributed to a softening real estate market, but can only be the result of the county's overassessment of a large number of single-family homes.

Applied to the total number of single-family detached homes in Fairfax County, the one-in-six rate found among January sales implies that about 25,000 homes were likely to have been overassessed in 1990.

Overassessment on this scale is unconscionable and should have prompted the county to review the assessment methods that allowed it to happen. Unfortunately, the low rate of appeals does not give the county a very strong incentive to do things differently. Worse, it may have been taken as a signal that the county can continue to overassess large numbers of homes without much resistance from homeowners.

To prevent repetition of what happened last year, two steps need to be taken to encourage legitimate homeowner appeals. One is for the county to make public its assessment methods in sufficient detail (for example, the models used and the geographical boundaries within which land values are assumed comparable) that homeowners can judge whether their own assessments are reasonable and fair or whether they have grounds for appeal.

The second step is to remove (through a change in state law) the risk that property owners currently face: If they appeal, the county may actually raise their assessments. Statistics may show that this does not happen very often in Fairfax County, but the threat in itself is a significant deterrent to homeowner appeals. Unsure in the first place whether they have a strong case for appeal, many homeowners feel they cannot afford the risk of a further increase in their taxes and therefore silently accept assessments they believe unreasonable. -- Dorothy and Alfred Tella are economists for Citzens for Sensible Taxation, a tax reform group in Fairfax County.