The U.S. Postal Service and its private development partners, bowing to strong opposition from preservation officials, have lopped one story off their plan to add 2 1/2 stories to the historic former main city post office adjacent to Union Station.

The revised proposal, to be heard Wednesday by the D.C. Historic Preservation Review Board, shaves the planned addition from 42 feet above the building's current roof line to 31 feet -- a height that will leave an addition of 1 1/2 stories.

In March, the review board unanimously rejected the conceptual plans for the $100 million project based on its objections to the 2 1/2-story addition, which one board member said would be "overpowering" to the surrounding area, particularly the elegant, old railroad station.

Dennis Wamsley, general manager of the real estate office at the postal service, said the revisions "represent the maximum changes we can make without harming our program. I don't think it's as good a plan {as previously proposed} . . . but we've tried to be responsive to the {preservation review board} concerns" regarding the addition.

The postal service has proposed renovating the former central mail handling facility for the District into a huge complex of 1 million square feet of private and government offices, as well as shops and a postal museum. But critics of the plan have argued that enlarging the 74-year-old beaux arts-style building would destroy its character and create traffic snarls in the already congested area a short distance north of the Capitol.

The prospective developers of the project this week criticized opponents of the renovation.

"The point of preservation is to preserve what was, not to oppose anything additional," said Stephen Porter, a Washington attorney representing the project's New York developers. "We're not destroying anything of historical importance, but preserving it."

While the new plan would reduce the height of the proposed addition, the developers would not lose any of the 250,000 new square feet of office space planned for the building. Instead, extra office space would be carved from a floor below the roof line that overlooks the building's courtyard.

Preservation activists this week had not yet seen the revised plans and declined to comment on the changes.

While new home sales rose 7.6 percent in April, James Fischer, president of the National Association of Home Builders, predicts that April and perhaps May will prove to be the last months of increased sales before the effects of higher mortgage interest rates take their toll.

Fischer said each 1 percentage point increase in mortgage rates changes the home-buying decisions of about 450,000 households in the new and existing housing market. "Some are priced out of the market, while others purchase lower-priced homes, increase their down payments or switch to adjustable-rate loans to keep the monthly payments affordable," he said.

Since the beginning of April, interest rates have risen from 9 percent to about 11 percent. On a $100,000 mortgage, the monthly payment totals $952 at 11 percent and $805 at 9 percent.

The Mortgage Bankers Association of America reports that the percentage of Americans late in their mortgage payments during the first three months of the year fell for the fourth consecutive quarter, the first time that has happened since 1964.

A total of 5.19 percent of the borrowers were 30 or more days late in the first quarter, compared with 5.38 percent in the last quarter of 1986. The percentage of loans entering the foreclosure process remained essentially unchanged -- 0.27 percent compared with 0.26 percent in the October-to-November period of 1986.