Arlington Manager Larry J. Brown has told the County Board that spending can increase 3.8 percent next year and allow the county to retain the current real estate tax rate of 97 cents per $100 of assessed value.

In a report released this weekend for the board to consider Saturday, Brown says the fiscal 1986 budget can be balanced with a 3.8 percent increase.

He is seeking approval from the board of preliminary spending guidelines.

While emphasizing that no figure has been set for salary increases for county and school employes, Brown's proposed budget guidelines calculate, for purposes of planning, a 3.8 percent raise. The figure is keyed to the current inflation rate.

Unlike recent years when most teachers got higher raises than other county employes, board members this year have indicated they want the increases to be equal.

The budget will not be proposed officially until next February and adopted in April, but the spending guidelines for developing it project a $216.6 million general fund to finance the operations of the county government, including $53.9 million for schools.

Mark B. Jinks, the county's budget director, said yesterday that the tentative school spending guideline provides the funds needed to maintain programs at their current levels.

Separate from that amount, Jinks said, is a new capital improvements fund of $1 million. The County Board had suggested an unspecified increase in school capital funds after a work session with the School Board last month on the schools' needs for improvements to physical facilities.

"The school system really has capital needs that have been neglected," former school board and current County Board member Mary Margaret Whipple said Saturday. "The amount set aside over the last several years has been far less than the county's pay-as-you-go" funds. The schools got an unusually high $800,000 in capital funds this year, but they have averaged closer to $400,000 annually, she said.

Still to be decided, Whipple said, is how much might be earmarked for program improvements the schools might request.

The School Board has been considering, for example, adding a seventh period to the school day next year, an action that Jinks said could cost between $750,000 and $1.5 million.

The extra class period is being examined as a way to meet students' needs to fit in more electives in a course load that is top-heavy with the mathematics and science requirements of the state's advanced diploma program.

The budget guidelines predict a $1.3 million deficit, but Jinks said that shortfall is expected to be offset when revenues are revised. The guidelines include a $2 million contingency account and $12.6 million for the county's share of the Metro transit system.

The community activities fund, programs and facilities jointly run by the county and schools, is tentatively set at $2.3 million.