A South Lake Union streetcar line proposed by Seattle Mayor Greg Nickels would increase the value of properties near the line by $70 million to $80 million, according to an appraiser's study commissioned by the City Council.

But neither Nickels nor the City Council is likely to ask property owners for more than the $25 million they've already offered to help build the $47.5 million line connecting downtown's Westlake Center to the Fred Hutchinson Cancer Research Center.

The study of streetcar benefits to nearby property owners by Bruce C. Allen & Associates, a Bellevue firm, is scheduled to be released today at the council's Transportation Committee meeting.

Initial reaction from large-property owners, council members and the mayor's staff was mostly upbeat. They said the appraisal means the city should be on solid legal ground in charging property owners near the line $25 million to help construct the 2.6-mile loop.

"You always want a significant margin of safety to insure yourself against possible legal challenges to assessments," said Councilman Richard Conlin, chairman of the Transportation Committee and a cautious supporter of the streetcar proposal.

City officials would ask property owners to pay for the streetcar through a special taxing mechanism called a local improvement district.

The city would assess property owners in the district a charge depending on how much property they own and how close it is to the streetcar line. Those within a block of the streetcar would pay more than those three to four blocks away.

While many property owners have told city officials they were willing to contribute, no specific amounts have been assessed and some property owners might balk at the assessments.

Conlin said he didn't see a need to charge property owners more. "We usually pay for transportation systems and don't ask for anything. These property owners came in and asked to pay."

Councilman Nick Licata, a streetcar skeptic, said he could not see his colleagues trying to extract more from property owners. "It's a political problem more than anything," he said. "The mayor has already cut a deal for $25 million. The people in the streetcar coalition feel that's their contribution and they're not going to budge."

The study looked at the value of property within five blocks of the proposed line. It then looked at the benefits of other streetcars around the country and projections that 1 million passengers a year eventually would ride the Lake Union system.

The study then concluded that property values near the streetcar line would increase to roughly $4.9 billion, after the $70 million to $80 million was added. It said the streetcar would benefit property owners because it would reduce auto traffic, cut transportation costs to residents and employers in the area, and would link South Lake Union to downtown and other transit systems.

Michael Mann, a strategic adviser to Nickels, said the $25 million offer by South Lake Union property owners represents the largest private contribution to a streetcar system in the country and is sufficiently generous.

Paul Allen's development company, Vulcan, is the largest property owner in South Lake Union with nearly 60 acres.

Vulcan has long advocated for the streetcar, saying it would spur redevelopment and create jobs while helping transform the neighborhood's low-slung warehouses into a more-urbane area like Portland's Pearl District, which has a streetcar.

The Seattle Times Co. also owns property in the South Lake Union area and has publicly supported the concept.

Vulcan executive Lyn Tangen said she did not think Vulcan or the pro-streetcar coalition it is a part of would be willing to pay more.

Conlin said the council could approve legislation this summer that would start the complex process of assessing property owners for the streetcar.

Bob Young: 206-464-2174 or byoung@seattletimes.com