Q I have just received a notice of assessment for my property in the District of Columbia. The District has determined that my property has increased significantly in value, and I believe that this assessment is much higher than the real worth of my property. What can I do?

A According to the D.C. Department of Finance and Revenue, more than 155,000 notices of proposed real property assessments for fiscal 1986 were mailed out to property owners last week. The assessment is used as the basis for determining the annual real estate tax bills.

Real estate taxes in the District of Columbia are paid twice a year. The District's fiscal year begins on July 1 and ends on June 30. The first half's taxes are due on Sept. 15 of each year and the second half's taxes are due by March 31. The notice of proposed real property assessment that you received will be the basis for your real estate taxes for the next year. Do not forget that the second half's taxes for fiscal 1985 are due by the end of this month.

Although the real property tax rate for fiscal 1986 has not yet been set, it is helpful to know what the tax rate was for this year. It may very well be that the D.C. City Council will use the same rate next year.

The tax rate for fiscal 1985 was:

* Class I, $1.22 per $100 of assessed valuation.

* Class II, $1.54.

* Class III, $2.53.

Generally speaking, Class I properties include improved residential property that has no more than five dwelling units and is occupied by the owner. It also includes owner-occupied condominiums, and cooperative apartments in which at least 50 percent of the dwelling units are occupied by the shareholders.

Class II is improved real property used solely for nontransit dwelling purposes, and also includes cooperative apartments in which fewer than 50 percent of the dwelling units are occupied by shareholders.

Class III includes any improved property that does not fall into either Class I or Class II.

It should be kept in mind that D.C. real property assessors based your proposed assessment on what they estimated your property was worth on Jan. 1. "In making this estimate, the assessors took into account information such as recent sales of similar properties, current building costs and any income that is produced, or might be produced, from real property," the District says.

The D.C. government increased assessments of residential properties for next year by an average of 4.3 percent, with some areas registering increases of 24 percent.

If you do not believe that the proposed assessment accurately reflects the value of your property, you have the right to file an appeal with the Board of Equalization and Review. Appeals must be postmarked or hand-delivered by April 15.

Before you decide to appeal, however, you should do a little homework. Examine the assessments of comparable properties. The assessment rolls are public record, and can be reviewed in the lobby at the District Building (1350 Pennsylvania Ave. NW), at the Municipal Center (300 Indiana Ave. NW -- outside Room 252) and at some libraries.

Next, you should contact the Valuation and Assessment Office (727-6460) and talk with the assessor who actually reviewed your property. He or she can tell you the methods that were used to assess the property. The assessor has the authority to adjust your assessment administratively if there is an obvious mathematical or data-processing error, if there has been recent damage to your property or if the property record kept in the assessor's office is incorrect.

If, on the other hand, the assessor does not adjust your assessment, or if you remain unconvinced of the accuracy of the results, you may file an appeal. You may obtain an appeal form from any D.C. public library; by calling 727-6860; or by writing to the Board of Equalization and Review, Room 2066, 300 Indiana Ave. NW, Washington D.C. 20021.

You can present the findings of a competent private assessor, a survey of recent sales of property comparable to your own showing market values more modest than those the city assigns, documentation of past trends of income to demonstrate a smaller rate of return than the official estimate would indicate, or lower official assessments for comparable properties. It must be pointed out that appeals based on tax rate, hardship or reasons other than market value will not be considered.

In due course, after you file your appeal, you will receive a notice indicating a time and place for you to appear before the appeals board. Although your presence is not necessary, it is strongly recommended that you appear in person and make your presentation.

Remember that the Appeals Board has the right to raise the assessment as well as lower it once you file your appeal.

My advice to anyone contemplating an appeal can be boiled down to one basic concept: Are you willing to sell your property for the amount assessed by the D.C. government? All too often, although the assessment may appear high, in reality it is much lower than you know the property is really worth.