Lawyers are asking the King County Superior Court to reject a well-worn, dilapidated 120-year-old Capitol Hill mansion recently approved as a Seattle historic landmark.

Saying that the decision has “prejudiced” their client by “causing the loss of a sale” and “substantially destroying the economic value of the property,” lawyers for the estate that owns the Sullivan House at 15th Ave and E Olive St. filed the lawsuit last month after the old mansion became one of the city’s most unlikely properties to go forward in the designation process that sets up controls and incentives on certain properties deemed worthy of preservation by a city convened board.

In the suit, lawyers for Elaine Thorson, the retired schoolteacher they say moved from California and plunged her life’s savings into buying out other heirs to her deceased aunt’s unique Capitol Hill apartment property, are asking the court to reverse the land use decision on the house and send the mansion back to Seattle Landmarks Board with direction to “reject the landmark nomination based on the severe economic impact such a designation will have (and has had) on the petitioners.”

Thorson stands to inherit 52% of any proceeds from the sale, her lawyers say.

According to court documents and as reported earlier by CHS, Thorson had a buyer lined up for the $2.4 million property before the landmarks designation process began.

According to a filing with King County for the property, Thorson had a “memorandum of agreement” with developer Michael Nelson of MRN Homes for the property. Nelson, according to the MRN website, has been “involved in purchasing, land development and remodeling of homes in Seattle since 1992.” Of the 11 upcoming projects listed on the MRN site, eight are for townhome projects. Nelson has not responded to CHS inquiries about the agreement.

Unlike the majority of landmarks proposals for Capitol Hill properties that are undertaken as part of a development process as developers lock down any preservation requirements or incentive opportunities before undertaking a project, the Sullivan House nomination was a much different affair.

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A neighboring architect banded together with others interested in preservation in the area to mount the successful — and seemingly grassroots — effort to win designation for the house. As he started the quest, architect and neighbor Jim Castanes acknowledged to CHS that his effort was probably a long shot and told us he was paying for and pushing the effort forward himself after watching other houses in the neighborhood demolished to make way for new multifamily housing. “There’s lot of houses that we wished we could act on,” Castanes said.

In their argument, Thorson’s lawyers say the city’s landmarks ordinance prohibits designation should not deprive any owner “improvement or object of a reasonable economic use” —

For precedent and a very local example, the court might only need to look a few blocks away where the Galbraith House has been demolished despite its landmarks status. The Capitol Hill Historical Society explored the process by which the landmarks board okayed the demolition of the Sound mental health-owned property at 17th and Howell.

As for 15th and E Olive St, lawyers for the plaintiff say their client has been left “destitute” by the landmarks decision:

Beyond the court proceedings, the property must also be approved by the Seattle City Council for its final protections. That process usually takes around a year before a council vote.