The property that would provide the bulk of a $20 million bequest to George Mason University is worth a fraction of that amount, according to government assessments, and the chances of improving the land's zoning are uncertain.

University officials and the executor of the estate making the donation want denser development than the single-family housing now allowed under regulations.

But neighbors oppose such a move, and because the 39-acre parcel straddles the line between Fairfax City and Fairfax County, both the City Council and the County Board of Supervisors would have to approve any changes. Last month, the City Council unanimously rejected a similar request and a county land-use committee likewise has recommended against any change.

Without the higher density zoning, the property is worth from $2.5 millon to $5 million, and the gift to George Mason could be substantially less than the $20 million that university officials announced on Monday would be forthcoming from the estate of the late Shelley Krasnow, a Northern Virginia businessman.

The money would be used to establish an institute for eminent scholars at George Mason.

Several civic activists said that, by announcing the gift, the executor and university officials are putting pressure on the council and the board.

"I think the $20 million value placed upon this gift is very, very conditional," said Sally Ormsby, a civic leader in the adjacent Mantua neighborhood. "It depends completely upon changing the comprehensive plan and a rezoning and . . . it appears that economic pressure is being put on the governments so they can realize a greater gift for the university."

But Mark P. Friedlander Jr., the lawyer acting as executor of the Krasnow estate, said the $20 million figure was a reliable estimate that the university can count on.

"I don't think it's all that speculative," he said. "The question is when. It's just not reasonable to figure that that land use would be expensive single-family dwellings."

Friedlander said that he expects as much as $15 million of the bequest to come from the sale of the wooded, hilly tract along Pickett Road between Routes 236 and 50 on the eastern border of Fairfax City; the rest would come from other properties in Prince William County.

Assessments conducted by city and county appraisers in January 1990 put the value of the Fairfax property at $2,484,700. However, Friedlander said a private appraisal he commissioned after Krasnow's death in September 1989 concluded it is worth $5.5 million without the higher density zoning.

Friedlander and University Senior Vice President J. Wade Gilley are trying to draw up a development plan that could mix single-family houses and moderately priced town houses on the property.

If they cannot then win support from the city and county, Friedlander said he would consider giving the property to the state, a move that would take the property off local tax roles and exempt it from local zoning laws.

"I don't intend to sell it as it's presently zoned because it's zoned at a use that's not economically practical," he said.

But neighbors are equally determined to keep out any development they deem as too dense, particularly the apartments originally proposed by Friedlander last year. "Obviously we'd like to keep it at the lower density," said Norman Neiss, chairman of the Providence District task force that rejected that proposal. "Undoubtedly, there's going to be some fighting along the line."