Loudoun County Administrator Philip Bolen, citing an unexpected surge in the county's growth rate, has proposed a $108.7 million budget that would increase county spending next year by a whopping 32 percent and raise real estate taxes on the typical house by 15 percent.

"We projected that 3,500 people would move into Loudoun County last year," Bolen said as he released the budget Thursday night. "Instead we added more than 5,000; that's 14 new people every day."

Booming development, much of it in eastern Loudoun, will give the county between $7.7 and $8.8 million in additional funds next year, the administrator said. But that will not provide for needed services, he said, as he recommended that the County Board raise the real estate tax rate from $1.13 to $1.16 per $100 valuation. That, coupled with increased assessments, would result in 15 percent tax increases for typical Loudoun houses, officials said.

The budget was greeted yesterday with some skepticism. "I've never known a budget that hasn't been cut," said Board Chairman James F. Brownell.

Bolen said his budget was "the first in Loudoun history to have nine figures to the left of the decimal."

Brownell said he was troubled that Bolen had called for a pay increase of 3.5 percent for county workers while the school board was calling for an 11 percent increase for its employes. "Otherwise it's a good budget."

Board Vice Chairman Betty Tatum said she was delighted that the budget would address some of the county's growth needs by adding six new positions in planning and technical services and adding a lawyer to update zoning ordinances. "These particular proposals answer a crying need in this county," she said.

Bohlen noted that commercial and industrial construction added $150 million to the county's assessments in 1985 and said Xerox Corp.'s Potomac Park development project in eastern Loudoun will have an estimated assessed value of $1.2 billion when completed.

But he warned of some "difficult times" until more revenue from new commercial and industrial properties is realized.

Current trends indicate 25,000 new residents could be added to the population rolls by 1991, he said. There are now 71,000 residents in Loudoun County.

No personal property tax increase is proposed.

Half of the budget, $56.4 million, would go for education. The budget also proposed $450,000 for the 3.5 percent salary increase but Bolen recommended that the county work week be expanded from 37.5 hours to 40. This would reduce overtime payments and improve productivity, the administrator said.

Bolen's budget also requested $670,000 for roads, water and sewers at the 92-acre site the county purchased last year with an eye toward building additional county offices. "The staff needs the space," Tatum said. "Some of these offices look like glorified closets."