Fairfax County's budget deficit next year will be up to $40 million larger than estimated earlier, Board Chairman Audrey Moore said yesterday, blaming reductions in state aid and the region's worsening economy.

The new deficit projection is in addition to a $60 million revenue shortfall that the board wiped out in August by ordering strict limits on county spending for fiscal 1992, which begins next July. The new estimate means that county revenue is expected to be as much as $100 million less than projected as recently as May.

Moore said a conservative estimate of the deficit the county faces is $25 million to $40 million. She renewed her call for the Board of Supervisors and school board to cut money from their budgets and hold it in reserve for next year.

The County Board "is going to have to make big cuts in 1992 or find more money in 1991" to carry over to the next year, Moore said. She repeated her promise to deal with the shortfall by cutting spending and not by raising taxes.

"People are fed up with taxes that are beyond a level that is reasonable to them," she said. "We have to tighten our belts, become more efficient and spend smarter."

That could be the rallying cry of the day for local, state and national governments, all of which are struggling to meet voter expectations for services and demands for tax relief at the same time that the economy appears headed into a recession.

Fairfax County Executive J. Hamilton Lambert has directed agency heads to trim 5 percent from their proposed 1992 spending plans this fall.

Moore said the board's decision on where to cut the budget should be guided by those proposals.

She said she would not support across-the-board cuts, reductions in areas that could affect public safety or debt service to pay off county bonds.

Moore said the county could lose as much as $15 million in education aid and $11 million in real estate filing fees that were expected from the state. In addition, she said, projections show that local real estate property taxes could fall $17 million short of expectation. All those shortfalls could add up to $43 million in the worst-case scenario.

Lambert said Moore's overall estimate was "in the ballpark," but said he would not comment on details. He said it appeared that increases in property assessments, which helped finance large budget increases in the last decade, would be less than 2 percent this year, while revenue from sales taxes and personal property taxes will be down significantly, based on the decline in new vehicle registrations.

Supervisor Thomas M. Davis III (R-Mason), who is considering running against Moore for the chairman's job in 1991, said he agrees that programs will have to be cut next year, but accused Moore of political posturing.

"If it's so bad, why are we running up a record amount of debt? The whole debt structure is making us all nervous, and those are programs that can't be cut," he said.

"She's started the {reelection} campaign and has repudiated her first two years in office {and} the taxes, taxes agenda," Davis said. "She's done a complete turnaround."

Moore has been preaching a message of fiscal prudence since it became apparent earlier this year that the region's slumping economy would cause a slowdown in county tax collections. She has taken a hard line against wasteful spending and tax increases, and even voted against the county budget this year because it included an increase in business utility taxes.

Moore's opponents charge that she only recently has shown signs of fiscal restraint. With a simmering taxpayer revolt in the county and elections 13 months away, they say Moore is trying to cloud her record as one of the region's most outspoken advocates of tax increases.

During her first two years in office, Moore took a high-profile stance in favor of increasing a variety of taxes to pay for transportation improvements.

In July, Lambert told the Board of Supervisors that if spending continued at its projected rate, the county would face a $60 million deficit in 1992 because tax revenue was falling sharply. The county budget was projected to increase 8 percent, and county spending on schools was expected to rise 10 percent.

In response, the board ordered that increases in county government spending be limited to 4 percent in 1992, and that growth in county funding for schools be limited to 6 percent. School officials say that would give them an overall budget increase in 1992 of 3.4 percent. Those actions eradicated the $60 million projected deficit.

House prices throughout the Washington area are flattening this year, which will be reflected in lower property tax revenue for local goverments next year.

In a recent regional review of real estate transactions, the real estate information company Rufus S. Lusk & Son Inc. found that the median price of existing and new single-family houses sold in the area in May 1990 was $196,005, a narrow 1 percent over the May 1989 price. The median price in April 1990, Lusk reported, was 5 percent lower than the year before.