The property of the United Nations is among the most valuable real estate in Manhattan and could prove to be a gold mine of residential development on the East Side were the world body to relocate, industry experts said.

“This is a spectacular site that developers would be eager to develop,” the head of planning for Rockrose Development Corporation, John McMillan, said. Rockrose is building a luxury residential development across the East River in Long Island City.

“It doesn’t get any better than this,” Mr. McMillan said.

“This site could fetch over $500 million, with residential and commercial developers alike foaming at the mouth to get what used to be an old pig farm,” a managing principal at Newmark, Scott Panzer, said.

Indeed, the seven acres of prime real estate facing the East River was once stockyards and farmland. The Rockefellers donated the property to the United Nations in the late-1940s and, as sovereign territory, the organization could sell it – for a world-beating price.

The iconic Secretariat Building, with its 360-degree views and proximity to the Midtown office district, would be ideal for residential use, experts said.

“This is an extremely valuable piece of property and would certainly be converted into residential space if the United Nations were to relocate,” said the head of the Real Estate Board of New York, Steven Spinola.

The Secretariat Building, with its narrow width, would be easily converted into residential space because its shape is conducive for constructing long hallways with apartments on either side, experts said.

“Every room in a residential development must have windows, and it is often hard to convert a traditional square building that has a large interior space into apartments because the interior is too far from the windows,” said one developer, who declined to be named. “The Secretariat has a depth of roughly 65 feet. That would eliminate this problem.”

The zoning of the U.N. site is also favorable for development, with a C5-2 designation that allows for commercial and residential development on the property, with a floor-to-area ratio of 10. That includes the surrounding greenery and parks, which would permit more intensive development of the site with relative ease.

“If the site were sold, it would not be difficult to build other buildings on it,” Mr. Panzer said, adding that some mixed-use development would also be likely for the area.

Another expert, the president of appraisal firm Miller Samuel, Jonathan Miller, said: “There is a lot of property surrounding the United Nations, and the value for a developer is the upside potential that they could build more developments there.”

The Secretariat Building is in need of renovation. Further, it is a designated landmark, which could complicate matters, because developers could improve the interior but would face legal hurdles in attempting to alter the exterior, Mr. Panzer said. There are also rumors of asbestos in the walls of the building, and of antiquated plumbing and electrical systems in need of replacement.

Despite those possible impediments to development, real-estate experts estimate a handsome profit margin for such an endeavor.

“I could easily see buying it for $600 a square foot, using an additional $200 a square foot to fix it up, and then selling it for $1,500 a square foot to make a nice profit,” the developer who requested anonymity said. That profit works out to 88%.

According to appraiser Miller Samuel, the average condominium price in the Midtown East-Turtle Bay area in the third quarter was $1.388 million, with the average price per square foot $939. The Trump World Tower at 845 United Nations Plaza, across First Avenue from the United Nations, is a likely “comparable” for what the U.N. property could generate. The August sale of a 2,016-square-foot, two-bedroom, 2.5-bath unit on a high floor of the building brought $2.325 million, or roughly $1,153 a square foot.

Sutton Place and Beekman Place also neighbor the U.N. site. The average price per square foot in the third quarter for those neighborhoods was $700, with an average sale price of $995,416. Those prices are lower than for the Trump skyscraper because the neighborhoods consist mainly of cooperative buildings, which sell for up to 30% less than condominiums, Mr. Miller said.

“If the land were to become available, it could create a tremendous boom along the East River, with high-end residential development and some mixed use,” said a senior fellow at the Manhattan Institute, Julia Vitullo-Martin. “The United Nations has eliminated development options for a huge section of New York.”