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. The D.C. Department of Finance and Revenue has just mailed out more than 155,000 notices of proposed real property assessments for fiscal 1987. 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 tax is due by March 31. The notice of proposed real property assessment that you received will be the basis for your real estate taxes next year. Do not forget that the second-half taxes for fiscal 1986 are due by the end of this month.

Although the real property tax rate for fiscal 1987 has not yet been set, it is helpful to know what the tax rate was for this year. It may well be that the D.C. City Council will use the same rate next year.

The tax rate for fiscal 1986 was:

*Class 1, $1.22 per $100 of assessed valuation.

*Class 2, $1.54.

*Class 3, $1.82.

*Class 4, $2.03.

Generally speaking, class 1 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 50 percent of the dwelling units are occupied by the shareholders.

Class 2 is improved real property used solely for non-transient dwelling purposes, and also include cooperative apartments in which fewer than 50 percent of the dwelling units are occupied by shareholders. Property that has no more than five dwelling units also is in this class.

Class 3 contains commercial (or business) property that houses transients. Included in this class are hotels, motels and inns.

Class 4 includes any improved property that does not fall into the previous classes.

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

If you do not believe that the proposed assessment accurately reflects the value of your property, you may 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 can be reviewed in the lobby at the District Building, at 1350 Pennsylvania Ave. NW; at the Municipal Center, at 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 had 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.

My advice to anyone contemplating an appeal: 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 really is worth. Benny L. Kass is a Washington attorney.