A crunch on open space in many rejuvenated cities has developers courting churches with multimillion-dollar offers to buy the air above them.

"In an urban area, air rights are just as much an asset as a piece of property," said the Rev. John Buchanan, pastor of Fourth Presbyterian on Michigan Avenue in Chicago. The church is working on a deal that could bring in $25 million.

From New York to Seattle, downtown congregations are striking deals worth tens of millions of dollars. Those willing to sell are often mainline Protestant churches saddled with aging buildings, growing deficits, and shrinking memberships.

A church that doesn't reach the maximum height allowed by zoning laws can sell the unused space to a developer, who can transfer that space to an adjacent building, and thus add stories to it. Churches can make millions off a "vertical asset" that would otherwise go unused.

On Manhattan's tony Park Avenue, the Byzantine-style Christ Church United Methodist is dwarfed by high-rise apartment buildings on the corner of East 60th Street. Christ Church negotiated a selling price of $430 a square foot-twice the going rate in New York's cutthroat real-estate market-for their 70,000 square feet of unused vertical space. The November deal generated $30 million for the church.

The church's pastor, the Rev. Stephen Bauman, said the money will fund ministry programs, including a public school in the South Bronx that has been "adopted" by the church.

The members of West-Park Presbyterian Church on New York's Upper West Side were offered $40 million for their crumbling building on Amsterdam Avenue, but decided to sell air rights for about $15 million. The money will fund repairs to the church.

"We wanted something more creative than finding ...