Conventional wisdom would tell you that the District of Columbia has the highest taxes in the metropolitan area.

That's true only if you make $50,000 a year or more, contends a new study of state and local tax burdens calculated by the D.C. Department of Revenue and Finance.

Alexandria and Fairfax County impose more taxes on a typical low-income family earning $10,000 a year, the study shows, and Alexandria and Montgomery County extract more than the District from middle-class families in the $17,000, $25,000 and $35,000 a year ranges.

The report issued last week confirms that at all income levels, D.C. taxes are higher than the average for the area. But the differences are not all that great except in the upper brackets, where the price of living in one of America's most livable cities is a couple of thousand dollars a year more than the price of life down in the boondocks of Loudoun County. Cheap at twice the price.

The tax burden study reveals the obvious truth that running governments in urban areas and service-demanding suburbs costs more than governing rural counties.

To the surprise of no one, Loudoun and Charles counties and Fairfax City are the lowest tax jurisdictions. Prince William County, though, is paying the price of rapid development in taxes as stiff as those paid by Prince George's or Fairfax residents at most income tax levels.

The study looked at personal income, real estate, sales and automobile taxes in the 11 major local jurisdictions. It looked at the taxes likely to be paid by a couple with one income, two kids in school and houses and cars supposedly typical of their income category. More about those assumptions later.

Although it compares gasoline tax rates, the study does not take into account the extra driving required to commute from the out-counties into town. Nor does it deal with the differences in government services and quality of life in various communities.

Once you cross the District line, the worst taxes are in Alexandria, though Montgomery County isn't much better. Alexandria's tax burden is the heaviest in three of the six income categories studied and is second in the other three. Montgomery County is second or third at all income levels except the lowest; people who earn $10,000 a year pay very low taxes in Montgomery, but few folks earning that little can afford to live there.

Tax-wise the best adjacent suburban county is Prince George's, which has cheaper-than-average taxes for every income level. A $75,000-a-year family would save more than $2,000 a year by moving from the District to Prince George's, and would pay $1,000 a year less in Prince George's than in Montgomery. A $25,000-a-year family would pay a couple of hundred bucks less in Prince George's than in the District.

These generalizations are drawn from 60-plus pages of charts, graphs and tables that tell most people more than they want to know about taxes. For businesses and individuals who want to calculate how their own finances might be affected by moving from one community to another, the detailed breakdown of tax levies in all the jurisdictions is an invaluable resource.

The one chart that provides the quickest reading on whether you're paying more or less than your neighbors is this breakdown of the average taxes paid by all local residents at various income levels: AVERAGE INCOME LEVELS INCOME . . . . . . . . . . . . .TAXES $10,000 . . . . . . . . . ..$873 $17,000 . . . . . . . . . .$1,475 $25,000 . . . . . . . . . .$2,278 $35,000 . . . . . . . . . .$3,248 $50,000 . . . . . . . . . .$4,854 $75,000 . . . . . . . . . .$7,329

Before you panic at being badly over- or undertaxed, consider some of the assumptions that D.C. tax collectors used in coming up with these figures. Some of the assumptions suggest the folks who did the study live in Fantasyland rather than in Washington.

Sales taxes are calculated from the amounts allocated for various income categories on the federal income tax return.

Income taxes are right off the local returns, using the average deductions found by the Internal Revenue Service on federal returns at each income level. The D.C. folks may have hurt their own case here by using figures for a one-income family, because the District's method of joint-separate returns favors two-income households.

Property taxes are figured on the assumption that the average family lives in the median-income house, and people who earn proportionately more move up to proportionately more expensive digs. Trouble is, that produces the assumption that a $75,000-a-year family in the District lives in a $229,000 house. That's a lot more house than most lenders would finance on that income at today's interest rates, and it tends to exaggerate the already high D.C. property tax burden.

Auto taxes are based on the assumption that even a $10,000-a-year family can afford a 6-year-old Toyota and that $50,000 a year will get you a four-door Volvo and a big Ford station wagon. Not in my neighborhood; nor will $75,000 a year let you drive the big BMW 528 and the Ford Fairmont the tax collectors think are possible. The auto fantasies tend to exaggerate the tax burdens in Virginia, which levies personal property taxes on cars.

By my calculations, the mortgage on a $229,000 house and payments on an $18,000 BMW would eat up three-quarters of the take-home pay of a $75,000 income, which doesn't leave much discretionary income for quiche and white wine.

But the flaky factors are applied across the board in the interest of fairness, and fairness is the one point on which the District scores highest in the comparison.

The tax burden in the District and Montgomery County rises consistently in step with incomes, paying homage to the principle of progressive taxation. A $10,000-a-year D.C. family pays 8.66 percent in taxes, while a $75,000-a-year family pays 11.77 percent; in Montgomery the rates escalate from 7.69 percent to 10.46 percent. Rates in Prince George's and Charles counties also move up progressively.

Virginia, on the other hand, generally has a less progressive tax system, and Fairfax County bears the embarrassment of having a regressive tax system that taxes the poor more heavily than the rich. A $10,000-a-year family in Fairfax pays 11.79 percent of its income in taxes (a higher percentage than even a $75,000-a-year D.C. family), while a $75,000-a-year Fairfax household pays at a 10.16 percent rate.

With a regressive tax system like that, Fairfax County leaders have a lot more to answer for than their counterparts in "high tax" Washington. At least the District taxes people who can afford to pay.