Homeowners in Shepherd Park NW, a racially integrated neighborhood nestle a long the District-Maryland line, had better be seated when they get their new real estate assessments in the mail next month. Their assessments have risen an average of 50 per cent from two years ago.

Property owners in the black middle class community of Michigan Park in upper Northeast and their white counterparts in upper Northwest Hawthorne along Oregon Avenue and Berkeley, cradled between Glover Archbold Parkway and MacArthur Boulevard, will receive increases ranging from 40 to 43 per cent.

In fact, of the 26 neighborhoods that the city last reassessed in 1975, 14 have assessments that are 30 per cent higher.

Many of the 26 neighborhoods were also hit with substantial assessment increases in 1975 over 1973, a general reflection of soaring real estate values in parts of the city.

Another 30 neighborhoods - the rest of the city that was reassessed just last year - may also be in for an assessment boost again this year if it survives a legal challenge currently being made in D.C. Superior Court.

The District government recently converted to a citywide annual assessment system, abandoning the blennial system under which the tax office had divided the city into two roughly equal property groups - "A" and "B" - reassessing each one every other year.

Attorney Gilbert Hahn Jr., leader of the court challenge, contends a previous court decision prevents the city from implementing the annual assessment system until next year. The city contends a 1975 congressional act requires adoption of the annual system now.

Superior Court Judge John Garrett Penn is expected to rule on the issue today.

Donald Beach, head of the assessment division of the city's Department of Finance and Revenue, attributes the higher assessments throughout the city to inflation and says they occur in neighborhoods that are popular and where many homes have changed hands in the past two years.

These are the areas where there is a high demand for housing and people are paying higher prices," he said.

City real estate is assessed at 100 per cent of market value - that is, the estimated amount it would sell for on the open market. The tax rate is $1.83 per $100 of assessed value0.

Ballooning assessments throughout the city this year could also mean a revenue windfall for the District government of $11 million if Hahn fails in his court challenge and the annual citywide assessment program remins in effect.

District finance experts originally estimated that the city's properties would yield only $12.4 million more this year than last year's total of $154 million. But after the assessors completed their recent citywide assessment, officials revised the additional yield to $12.4 million.

This additional yield amounts to an $11 million surplus over projected city budget needs. If sustained by Judge Penn, the mayor and City Council would have the option of keeping the surplus in the city treasury or returning it to the property owners by lowering the property tax rate by 10 to 15 cents.

The "Group A" properties - affecting about 75,000 taxpayers - were reassessed last year and, according to Hahn, should not be assessed again until next year. At that time, he argues, the regular two-year assessment cycle will be completed, and the city can convert to the annual citywide system then.

If Judge Penn rules that only "Group B" properties - involving about 61,000 taxpayers - should be reassessed this year, the city will realize only about $2 million in windfall revenues, instead of $11 million, tax officials estimate.

If Penn upholds the current citywide assessments, LeDroit Park, located just south of Howard University, will receive the largest increase of the Group A communities - an average of 34 per cent. Mayor Walter E. Washington lives in LeDroit Park.

Hahn has calculated that the higher assessments will cost each taxpayer in Group A an average of $120 more in property taxes this year than last year. The additional cost to taxpayers in Group B will be $237, he estimated.

District residents usually receive their reassessment notices in early February, but the notices have been delayed this year by the court action. The Washington Post obtained the new neighborhood assessment figures from the court records.

Notice of the new assessments will go out next month but will not be reflected in the taxpayer's bill until at least the Sept. 15, 1977, billing. Hahn hopes through his court action to delay the new assessment for Group A taxpayers until Sept. 15, 1978.