A real estate company is being blocked from redeveloping two Upper East Side tenements by a state court’s ruling that they are historic landmarks.

The decision caps a two-decade tug-of-war in which the buildings were granted landmark status, lost it, got it back with the City Council’s help, and most recently faced the prospect of losing it again. The tenements were built in 1915 as part of a model complex that aimed to improve the quality of “affordable” housing.

The 15-structure development, occupying a block on York Avenue between 64th and 65th streets, was first designated as a landmark in 1990. In a compromise with owners four months later, the now-disbanded Board of Estimate stripped two of the buildings of their status.

The Landmarks Preservation Commission restored the designation in 2006, and the City Council backed it by a unanimous vote. The owner sued in state Supreme Court but lost in the decision announced yesterday.

The ruling allays fears that the owner, Stahl Real Estate, would tear down the tenements to make way for a pair of glass office towers.

“This ruling is a big victory for our community,” Council Member Jessica Lappin, who serves the Upper East Side and fought to protect the buildings, said in a statement. “The Council righted a wrong by overturning the Board of Estimate’s bad backroom deal and we are thrilled that the court upheld our work.”

The tenements were constructed by the city and Suburban Homes Co., which was founded by a group of prominent philanthropists. The company gave up profits to provide the working poor with housing that exceeded standards of comfort at the time.

In her decision, Judge Emily Jane Goodman noted the “ironic effect” of the idea of building a 28-story tower in a complex that was historically significant for its access to light and air. The six-story buildings have been described as national models that helped spawn the federal housing movement.

Some of the tenements’ architectural details have been altered, including their original paint color and façade material. The owner admitted to making those changes simply so it could argue that the buildings lack value as landmarks, according to the court’s written opinion.

The court’s decision “will make owners throughout the City think twice before removing certain features from their properties in an effort to stave off landmark designation,” the Landmarks Preservation Commission chairman, Robert Tierney, said in a statement.

A representative for Stahl, Brian Maddox, noted that Judge Goodman cited the buildings’ history and structure — rather than their architectural detail — in concluding that they are landmarks. The company is considering an appeal, Mr. Maddox said.