Through much of the late 1990s, the Fairfax County government financed a large part of its operations on the ever-rising tide of real estate tax revenue generated by the county's ever-sprouting office buildings, which were being thrown up so fast that Fairfax soon had more commercial space than such cities as Boston, Philadelphia, Atlanta, Denver and Houston.

Commercial property assessments are based on the income-producing value of a building, and so, as the technology and telecommunications firms rapidly expanded, the commercial real estate tax base did too. At the same time, residential real estate values increased more modestly.

But now, as the economy plods along in the aftermath of corporate bankruptcies and downsizings in the county, the trend has been reversed. After the most recent reassessments of the nearly 332,000 properties in the county, assessors said the value of residential properties had increased by 14.55 percent, while that of commercial properties had dropped 2.94 percent.

In the county ledgers, that means that while just two years ago commercial properties yielded 28.9 percent of county real estate tax revenue and homeowners the rest, now the tally shows building owners bearing 23.2 percent of the real estate tax burden and homeowners the remainder. Homes have a total value of $99.2 billion in the county, up from $84.7 billion a year ago, but commercial property dropped in value from $30.2 billion to $30 billion.

In practical terms, that has left Fairfax officials answering complaints from angry homeowners facing higher real estate tax bills even though the county is proposing cutting its real estate tax rate from $1.21 per $100 of assessed valuation to $1.19.

Supervisor Gerald E. Connolly (D-Providence), who is the probable Democratic nominee for chairman of the county board in next fall's election, said the vast increase in homeowner assessments over the last few years at the same time commercial properties were increasing modestly "puts us in a terrible vise. The state gives us no tool other than the real estate tax to pay for government. It's not acceptable and it's not sustainable to place so much on the backs of homeowners."

He said that when the commercial real estate market burst in the early 1990s from overbuilding, the commercial share of the county real tax collection totaled 19.04 percent. "We've lost all the gains and we're virtually back to where we were," Connolly said. "Last time it took five or six years to recover."

It could well take that long again, he said, but he held out hope that "if this is a brief war and the economy rights itself, it may be positive." But he said the chief necessity is winning state approval for the county to duplicate the rights of cities in Virginia to be able to levy higher taxes on meals, cigarettes, event admissions and hotel rooms in order to lower the real estate tax burden on homeowners.

Don Deal, the county's assistant real estate tax administrator, said that a commercial assessment is computed by first determining the "potential gross income" of a building based on the amount of square footage it has available for lease and then subtracting the vacancy rate for the submarket in which the building is located, not the actual vacancy total in the property.

In that calculation, he said, real estate taxes are thus still owed on vacant office buildings, but reduced to the extent that there are vacancies in any of the county's 21 submarkets. The Fairfax office vacancy rate has jumped in recent months, and now amounts to nearly 20 percent; it's even higher in some submarkets. Deal said it is 16 percent in the key Tysons Corner market and 15 percent in Reston, another major office market.

-- Kenneth Bredemeier

An empty office building last summer in the Westfields area of Route 28. As commercial values drop, homeowners' portion of the real estate tax burden has increased. Supervisor Gerald Connolly notes the rise in homeowner assessments.