Loudoun County has frozen government hiring as a first step toward averting a nearly 10 percent budget deficit next fiscal year, potentially the largest percentage shortfall among Washington area governments.

County officials said yesterday that as many as 88 vacancies, representing 7 percent of the work force, will not be filled in light of the county's dire economic outlook. Officials hope to trim $6 million to $10 million from this year's $273 million budget to cushion the impact of a bigger crunch in the 1992 spending plan, which will start next July.

The move mirrors cutbacks underway or being considered in many jurisdictions in the Washington area as the sour economy pinches local budgets.

The Loudoun hiring freeze could save roughly $2 million this year and will be followed by more difficult measures in coming months, officials say. County budget officer John Wells said the 88 positions "span the whole range of departments" and that some jobs, such as in public safety, may be exempted from the freeze. He said the school system will examine its own cuts.

A sudden downturn in real estate assessments, linked to the rapid cooling of the once-hot real estate market, will cut property tax revenue as much as $5 million next year, officials say. Those taxes rose as much as $20 million a year in the late 1980s; the county budget grew 30 percent this year.

Declines in other revenue, including state money, and increased costs of maintaining basic services are also cited by Loudoun officials for next year's potential $28 million to $34 million shortfall.

Debt for schools, libraries and other construction projects constitutes an increasing part of the county budget, officials said.

Among the biggest uncertainties in the budget picture is whether the Board of Supervisors will increase the real estate tax rate in 1991, an election year.

"We're going to have to show real clearly that the county has made extensive cutbacks" in spending before approving any tax rate increase, said Board Vice Chairman Charles A. Bos (D-Leesburg).

"I don't think it will be a big tax increase," said Supervisor James F. Brownell (R-Blue Ridge). "Industry is definitely laying off people. I don't see how we can avoid it."

The 14,300-student Loudoun school system is feeling pressure from several directions, including the prospect of deep state cuts. The Board of Supervisors has asked the School Board to trim at least

$3 million from its current budget and has warned that county government funding won't increase substantially next year.

In addition, some supervisors have questioned the need for a

$34 million high school, which the School Board plans to open in the mid-1990s.