Residential real estate assessments in the District will increase by an average of 13.5 percent for the tax year beginning July 1, more than twice the previous year's increase, and some property owners have reported individual increases of more than 39 percent.

The new assessments, mailed late last week to the District's 155,700 property owners, do not mean an automatic increase in tax bills because the D.C. Council has the power to lower tax rates. Such a cut has not been made since 1979, but several council members said last night they are considering proposing one this year.

"I think it's an under-the-table tax increase," said council member Carol Schwartz (R-At Large). "The mayor says he is not increasing taxes on real estate, but then increases the assessments, which then causes tremendous tax increases. So who is he kidding?"

Mayor Marion Barry was out of town and unavailable for comment last night, and city finance officials declined yesterday to answer questions about the new assessments, which are calculated annually based on market conditions. A statement issued with the statistics said the increase "reflects sustained lower mortgage rates and the resulting improved residential sales activity in the District."

The 13.5 percent increase in District assessments, which compares with 5.5 percent in the previous year, mirrors similar increases reported in Northern Virginia. In Fairfax County, the average single-family house assessment rose 14 percent; in Arlington, the average assessment was up 16 percent.

Fairfax County has approved property tax rate reductions each year since 1984, but officials there have said a substantial reduction in the coming year is unlikely.

In Montgomery County, the average assessment increase on a single-family house was 4.9 percent. Property tax rates in the county were increased last year, but a reduction has been proposed for this year.

The largest percentage increase in the District, according to the city's figures, is in the Hawthorne area in upper Northwest, where the average increase was 29 percent. By contrast, there will be no increase in Fort Lincoln in far Northeast.

Among the city's most affluent neighborhoods in Northwest, the largest average dollar-amount increases in assessments are in Spring Valley, where the average rose almost $65,000 -- from $385,826 to $450,800 -- and Massachusetts Avenue Heights, where the average was up more than $66,000 -- from $599,300 to $666,787.

Council member Betty Ann Kane (D-At Large), who said her office has been swamped with calls about the new assessments, said she will fight to roll back tax rates to cushion their effect. However, she said there would be a battle looming over tax rates because the city is facing a budget crunch in the coming year.

Council member Jim Nathanson (D-Ward 3), whose own assessment went up by 27 percent, said, "I think the council is obligated to do something to mitigate the impact of the severity of these increases."

Nathanson said the council could adjust the tax rate in July or place a ceiling on the increase the taxpayer would be obligated to pay.

Commercial property assessments in the District showed even greater increases. The average increase was 18.9 percent, with property in many neighborhoods increasing more than 25 percent. The average commercial increase was highest in the downtown's central area, where assessments rose 39.3 percent.

Barry, citing budget shortfalls in the current and coming fiscal years, proposed last month a five-year $248.8 million income tax increase.Council Chairman David A. Clarke questioned last night whether the income tax increase will be necessary in light of the sharp rise in property assessments that will bring added revenue. He said the city's assessment figures indicate "there should be a reduction in the property tax rate, an avoidance of the requested income tax increase or a combination of both."

Council member Charlene Drew Jarvis (D-Ward 4) also said the figures indicate there may not be a need for an income-tax increase.

Property owners can appeal their new assessments to city tax assessors through a procedure known as the "stipulation process." Assessors have authority to roll back assessments in some cases. Property owners also can file formal appeals to the District's Board of Equalization and Review, an independent agency.

Last year, more than 1,300 property owners -- 53 percent of those who appealed -- won rollbacks totaling more than $20.8 million.

.............PRESIDENTIAL ASSESSMENT INCREASES*...................

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NEIGHBORHOOD..........% RISE...NEIGHBORHOOD...........% RISE....

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1. American Univ. Park..20.1...29. Kalorama..............9.3....

2. Anacostia............10.0...30. Kent.................24.0....

3. Barry Farms...........4.8...31. LeDroit Park..........8.7....

4. Berkley..............13.8...32. Lily Ponds...........12.7....

5. Brentwood.............0.7...33. Marshall Heights......6.2....

6. Brightwood............8.2...34. Mass. Avenue Height..10.1....

7. Brookland............10.5...35. Michigan Park........10.5....

8. Burleith..............7.9...36. Mount Pleasant.......14.4....

9. Capitol Hill.........17.0...37. North Cleveland Park.19.4....

10. Central.............13.1...38. Observatory Circle....7.6....

11. Chevy Chase.........18.7...39. Old City No. 1........9.0....

12. Chillum.............18.1...40. Old City No. 2.......16.7....

13. Cleveland Park......26.5...41. Palisades.............9.6....

14. Colonial Village....14.5...42. Petworth..............9.5....

15. Columbia Heights....14.4...43. Randle Heights........7.0....

16. Congress Heights.....1.2...45. R.L.A. (NW)...........2.7....

17. Crestwood...........20.1...46. R.L.A. (SW)...........0.7....

18. Deanwood.............8.1...47. Riggs Park...........11.1....

19. Eckington............7.3...48. Shepherd Park........18.0....

20. Foggy Bottom.........8.1...49. 16th Street Heights..22.0....

21. Forest Hills........11.7...50. Spring Valley........16.4....

22. Fort Dupont Park.....6.7...51. Takoma Park...........7.9....

23. Foxhall.............15.9...52. Trinidad..............3.0....

24. Garfield............15.3...53. Wakefield............12.4....

25. Georgetown..........15.4...54. Wesley Heights........8.4....

26. Glover Park.........14.0...55. Woodley..............14.7....

27. Hawthorne...........29.0...56. Woodridge............24.2....

28. Hillcrest............7.8...66. Fort Lincoln..........0.0....

*Numbers refer to neighborhood codes that appear on tax bills. SOURCE: D.C. Government.