GSA-inspired challenges to the tax assessments of a dozen downtown office buildings have resulted in reductions of more than $19 million, which will cost the District more than $400,000 in revenue this year.

Involved were buildings where the federal government leases large amounts of space and under terms of its leases must pay a pro-rated share of the buildings' taxes. The General Services Administration went after the assessments here in a pilot program to see if such challenges would make an effective cost-cutting device.

The reductions, totaling $19.38 million on nine buildings--there was no reduction on three--will save the federal government $386,937 this fiscal year. GSA will award building managers an additional $97,475 as a contingency fee for helping with the challenges.

"I am extremely happy," said John L. Stanberry, director of public utilities in GSA's Public Building Service. "I was amazed at how successful we were since we were originally hoping we'd win on about 50 percent of the cases."

The pilot program was the idea of GSA Public Building Commissioner Richard O. Haase, a former commercial appraiser in Washington. Haase said he often found the District's assessments "arbitrary" and thought that challenging them through "an absurdly simple process was too easy to pass up."

Jeffrey L. Humber, the District's director of finance and revenue, said that the city receives $360 million a year in commercial and residential property taxes and the $400,000 is "obviously money, but not a lot of it."

GSA screened each of the 112 privately owned buildings it occupies in the city, initially settling on 27 targets. That list eventually was whittled to a dozen buildings.

In the pilot program, at least 80 percent of the office space in each of the buildings had to be occupied by federal employes and it had to cover at least 100,000 square feet. In the future, GSA will reduce that mark to 51 percent federal occupancy, drop the space requirement to 50,000 square feet and try to run the program nationwide.

Humber said he was "watching" GSA's pilot program, but had not reviewed its track record. He also said he was not overly concerned about GSA plans to extend the program next year to additional buildings.

"You literally cannot extend the ratio of success" tax reductions in 75 percent of the GSA inspired challenges , Humber said. "You cannot assume that the circumstances will be the same."

City officials annually estimate how many assessments will be reduced by the Board of Equalization and Review, but could not readily provide statistics as to the impact of GSA on that mark this year.

"If we were to lose more government cases in the future and win more nongovernment cases, it would all equal out at the board," Humber said.

City officials added that, in budgeting each year they make allowances for successful assessment challenges. They said reductions by the board this year were substantially less than that allowance, so there will be no adverse impact on the city's tax revenues.

Haase said initially the pilot program would be expanded to cities where GSA leases large quantities of space, such as Los Angeles, San Francisco, Atlanta and Houston. So far, he said, only one federal employe has worked part time with one supervisor over a three-month period to turn up the $289,462 saving for taxpayers.

"That's a pretty good return for the dollar, I would say," Haase said.

Overall, the Equalization Board knocked down the assessed value of the nine buildings from $128.5 million to $109.1 million for the 1984 tax year. GSA did the best working with Westwood Management Inc., the building managers of Columbia Plaza, 2401 E St. NW. There the appraisal was for $25 million, but the challenge got it knocked back down to the 1983 level: $19.5 million. GSA will save $117,500 in tax payments on that building alone.

The federal government also saved more than $100,000 working with Shannon & Luchs, which manages the Small Business Administration's headquarters building at 1441 L St. NW--known as the Imperial Building.

Stanberry said some building owners initially contacted resisted participating because of fears that the city would increase the tax assessments.

"Fortunately, that didn't happen and I think the results on the nine buildings will send a signal to other building owners to work with us on this," Stanberry said.

Haase said that leasing officials have been told that it will be part of the agency's policy to require building owners to release operating income statements to GSA annually so agency officials can estimate whether any assessment increases are fair. In the pilot program, GSA sought that information but often was turned down by building owners who considered the information privileged.

"We don't want to go in and fight without the cooperation of the building owners," Haase said. "That's essential."

Stanberry said computer software is being developed to help agency officials cheaply monitor the appraisals.

"This is an every-year review process," Haase said. "There is every likelihood that the city will come in with appraisals next year that undo what we did this year."

Haase said he had heard that the challenges by GSA amounted to more than half of the reduced assessments awarded by the Board of Equalization this year. Board officials, including Chairman Sam Reynolds, did not return repeated telephone calls.