In the Metro section yesterday, a chart about area real estate taxes should have explained that property tax assessments in Maryland jurisdictions such as Montgomery and Prince George's counties are made differently than in Virginia and the District of Columbia, and that therefore tax rates are not directly comparable. In Maryland the assessment is 43.47 percent of a house's market value, not full market value, as in the local Virginia jurisdictions and the District. For example, in Fair- fax County, where fiscal 1987 property taxes will be $1.35 per $100 of assessed value, taxes on a $100,000 house would be $1,350. Montgomery County's fiscal 1987 average tax rate of $3.14 would be assessed against only 43.47 percent of a $100,000 house, resulting in taxes of $1,365. The Montgomery and Prince George's tax rates in the chart also include the basic county tax, plus additional charges for items such as parks and plan- ning and state property taxes.

The Montgomery County Council yesterday trimmed 6 cents off the residential property tax rate, but steady increases in assessments will add $25 to $30 to the average annual tax bill, county officials said.

The average property tax rate for county homeowners was reduced in a unanimous vote from about $3.20 to $3.14 per $100 of assessed value, but the rising assessments will increase the average tax bill less than 3 percent, according to Jacqueline Rogers, county budget director.

County officials estimated that the tax rate set yesterday would generate enough revenue to leave a $20.4 million surplus after funding the $1 billion budget for fiscal 1987 passed earlier.

Montgomery's actions follow similar moves by most other local jurisdictions, which this year have generally set the same or slightly lower property tax rates and relied on rising assessments to expand their budgets. However, the District of Columbia government for the first time in seven years took measures yesterday to reduce property taxes.

The most controversial debate on the property tax -- a quibble over pennies that would raise millions of dollars -- centered on how much to raise the county's current $2.040 general tax rate, one of two taxes levied on all county residents that is included in the overall property tax rate.

The council's unanimous vote to increase it about 4 cents to $2.078 from $2.040 was a victory for County Executive Charles W. Gilchrist who opposed a recommendation from the council's legislative advisers to raise it to $2.11.

Gilchrist originally proposed that the county budget include a surplus of $18.5 million. But the council's legislative advisers recommended the higher tax rate to build up a surplus of $25 million in an effort to offset possible state cutbacks or federal cutbacks mandated by the Gramm-Rudman-Hollings budget control law.

"We need to be ready for a rainy day with Gramm-Rudman," said council member David L. Scull.

Under Gramm-Rudman, if Congress and the president fail to agree on budgets that meet certain deficit reduction targets each year, across-the-board cuts would automatically be triggered in a number of federal programs until the nation's almost $200 billion annual deficit is eliminated in 1990. The law is being challenged in the courts.

The council's legislative advisers based their recommendation on an April report on possible effects of Gramm-Rudman-Hollings by a county task force, headed by Stuart E. Eizenstat, who served as President Carter's chief domestic adviser.

Gilchrist praised the council action yesterday, saying that more of an increase would have been "unnecessary and irresponsible."

"There was absolutely no excuse for increasing the surplus" in the county budget, Gilchrist said. "To have charged the taxpayers with a bigger surplus would have invited unnecessary election-year spending on the part of some of the council members."

With yesterday's vote, the county's overall property tax rate decreased for the sixth year in a row. That was due primarily to the good performance of other county revenues such as income taxes and sales taxes, good investments and the county's ability to hold expenditures down, according to Arthur W. Spengler, the council staff director. The overall tax rate decreased because of drops in tax rates in the county's special taxing districts, such as the recreation, suburban, sanitary, storm drainage, Maryland-National Capital Park and Planning, Consolidated Fire, Kensington Fire, and Rockville Fire districts.

In addition, there was a two-cent decrease in the mass transit tax, required of all county homeowners.

The mass transit tax and the general tax are levied on all county taxpayers. But the total property tax rate differs from area to area, because of varied recreation, sanitary or fire district taxes.

Tax bills will increase less than the 3.7 percent inflation rate in most areas of the county except Rockville and Damascus, according to Rogers.