The Falls Church City Council last week voted to keep its present real estate tax rate, the lowest in the Washington area. But residents will have higher tax bills anyway because property reassessments increased about 5 percent.

The average single-family home in the city was valued at $111,000 this year and carried a tax bill of $1,065.

The council also voted to require future tax bills to be paid in two installments, Aug. 5 and Dec. 5, instead of one payment due in December. Most other Northern Virginia jurisdictions require two payments to avoid summer fund shortages. Notices of the new billing procedure and taxes due Aug. 5 will be sent out just prior to the July 4 holiday weekend, city spokeswoman Annette Dennis said.

The $13.3 million fiscal 1983-84 budget, about $900,000 more than the current fiscal year, is about 7.5 percent more than this year's budget for general operating funds and just more than 2 percent higher for schools.

City Manager Harry E. Wells had recommended a 3-cent increase in the property tax, but the council was able to avoid the increase because the school system now predicts it will have a $40,000 surplus to carry over into the next fiscal year beginning July 1.

A $45,000 decrease in Metro bus and subway subsidies also is now predicted, and the council voted to save $65,000 by leasing city computer equipment instead of buying it.

No other city taxes or fees were raised.

The city property tax rate of 96 cents per $100 of assessed value has been the lowest in the region for several years. Arlington County has had the next lowest rate, 98 cents per $100, but the County Board voted last month to raise that 99 cents.