Loudoun County homeowners will have to pay part of their property taxes earlier next year under a plan approved by county officials. The plan will result in a cash windfall for the county of almost $50 million.

"There's going to be a financial hardship on some folks" to meet the accelerated payments, Supervisor Thomas S. Dodson (D-Mercer) said Tuesday night before the board voted 5 to 2 to replace the annual billing with twice-a-year collections.

The change, expected to be approved by the General Assembly, would bring Loudoun in line with all other major Northern Virginia governments, Loudoun officials said. The change calls for half of the annual Loudoun tax bill to be paid in June and the other half in December; currently, the whole bill is due in December.

As a result, the county would collect 1 1/2 years' worth of taxes in the budget year that begins on July 1: a full year's worth in December 1991 and six months' worth the following June. The windfall will be $45 million to $50 million.

Dodson, who opposed the change, said, "Some folks have to go out and borrow money to make that payment now."

However, Vice Chairman Charles A. Bos (D-Leesburg) argued that twice-yearly tax payments "have worked well in other counties . . . . It's a matter of folks getting used to it."

Collecting taxes only in December is an outdated practice "based on an agrarian economy," added Supervisor Steve W. Stockman (R-Broad Run).

Even though the county faces a projected $31 million deficit, supervisors said they won't use the windfall to solve its fiscal woes.

"I don't see legally how this board could commit those funds," said Supervisor Ann B. Kavanagh (D-Dulles). She and other officials suggested that the windfall could be put in reserve or used for major construction projects, such as a new high school.

County Administrator Philip A. Bolen, under orders to come up with a plan that doesn't raise taxes, is predicting massive cuts in county services and some layoffs.

The budget picture is exacerbated by declining land values. Although residential assessments are expected to be almost unchanged on average, commercial and industrial land values are plummeting. The net effect is that homeowners will bear a larger share of the overall tax burden, because all real estate must be taxed at the same rate, officials say.

Rapidly rising assessments, fueled in part by land speculation, fed increasing Loudoun budgets in the late 1980s. Even if the $45 million-plus windfall were used to eliminate the fiscal 1992 deficit and keep the tax rate low that year, the budget must shrink significantly for several years, Bolen says.

The Board of Supervisors, which faces elections this November, will approve a fiscal 1992 budget this spring.

The supervisors also voted this week to prorate personal property taxes, such as the levy on cars, beginning in 1993. Loudoun Treasurer George W. Titus said he will advise landowners and mortgage companies of the real estate tax billing change before it takes effect.