The Howard County Council adopted a $286.4 million operating budget yesterday that will increase spending by 18 percent while trimming the property tax rate for the first time in four years.

The council cut County Executive Elizabeth Bobo's proposed fiscal 1991 budget by $1.95 million to reduce the tax rate by four cents to $2.45 per $100 of assessed value.

Nevertheless, the owner of an average-priced $170,000 house will have a property tax increase of about $49, to $1,848, because of rising assessments. Without the rate cut, the tax bill on an average house would have risen $69.

"We did not cut any essential services," said council member Angela Beltram (D-District 2).

But member Charles C. Feaga (R-District 5), who unsuccessfully sought to use any savings to reduce the county's debt, predicted the election-year tax rate cut would be short-lived. "I have an idea we'll be raising {the tax rate} next year," he said.

Member C. Vernon Gray (D-District 3) disagreed. He said he would not be surprised if the county ended fiscal 1991 with a surplus larger than the $25 million expected this year. "The revenue estimates have always been very conservative," Gray said.

Bobo tried to take the "long view" in proposing no change in the tax rate, County Administrator Buddy Roogow said. "We know state reductions in education {aid} are coming. But we will certainly survive the cuts."

It is not known whether the county will begin curbside leaf collections this year, he added.

Adoption of the budget followed a week of wrangling between the council and Democrat Bobo. Council members, intent on cutting the budget to reduce the tax rate by four cents, had threatened to carve deeper if the county executive did not approve their request to add about $70,000 for various programs. The council must have the county executive's approval to in- crease spending for anything other than education.

Eventually, the council got part of what it sought. Bobo agreed to add $10,000 to the $70,000 she had proposed for helping foreign-born newcomers to the county. She also added $10,000 to extend the hours of a nutritional program for the elderly.

Council members then agreed not to cut $24,524 for hiring an aide to monitor state government. They also returned about $122,645 they originally had cut from a contingency fund.

In addition to adopting an operating budget, the council approved a $185 million capital budget for fiscal 1991 that is a 49 percent increase over current spending. That budget calls for construction of three elementary schools, a new police and fire complex and a library in east Columbia. It also sets aside money to expand a waste-water treatment plant and buy development rights on farmland.