Prince William County budget officials have proposed that the Board of Supervisors defer a proposed cut in the county's real estate tax rate and increase overall county spending by a "modest" 3.6 percent next year to $106.5 million in fiscal 1984.

The budget proposal, subject to approval by the board, calls for the county to retain its current real estate tax rate of $1.42 per $100 evaluation instead of continuing with the board's previously announced plans for a two-cent cut in the rate.

Budget officials said yesterday the change was necessary because the assessed value of both homes and businesses in Prince William rose by approximately 2 percent this year, compared with rates of 10 to 12 percent in recent years.

To make up for the lower growth in assessments, the county will have to increase some user fees and dip into a reserve contingency fund, said Budget Officer Blaine Aikin. The proposed budget calls for a $670,000, or 16.9 percent, increase in county fees for car registration, building permits, dog tags, marriage licenses and other services.

County Assessor Ben Kelsey said Prince William has fared better than neighboring jurisdictions this year because the continuing construction of town houses, condominiums, houses and businesses in the county, which has been one of the fastest growing suburbs in the region.

The proposed budget for the fiscal year beginning July 1, would not eliminate any services, but Aikin said some departments, such as public works and parks and recreations, will have cuts from four to six percent. "It will be a year of restraint," he said. "It is a modest budget because the general economy has a slower rate of growth."

Last year, bolstered by a 10 percent increase in property assessments, the county supervisors cut the real estate tax rate by 3 cents and said they would cut another 2 cents this year. The supervisors are scheduled to hold several public hearings on the budget before they formally adopt it.

Supervisor Donald L. White, who represents the Gainesville area, said the promised tax cut may be salvaged yet. "I haven't seen a budget yet that couldn't be cut further," he said.