If the United Nations wants to fix up its headquarters at Turtle Bay, it should pay for the renovations itself. The world body has asked the American taxpayer to put up more than $1 billion to shore up the Secretariat, fix the plumbing in the General Assembly, and improve security. The congressional General Accounting Office concurred in 2001 that the buildings need repairs.

By offering a 30-year, interest-free loan of $1 billion, the United States will be forgoing at least $500 million, and probably more, in interest payments. Those who favor the deal argue that this is traditional — after all, the treasury gave a similar, $65 million loan in 1948 to erect the buildings in the first place.

But this isn’t 1948. Europe and Japan, then in ruins, now rival this country for economic production. For the United States to offer free money on that scale would essentially be a free ride for the other rich nations of the world, and they can afford it. What’s more, the U.N. has strained in recent years to demonstrate its independence of, and defiance to, United States leaders. There’s no reason they should expect our largesse.

The U.S. already supplies some 25% of the United Nations operating budget, and the most that would be reasonable would be for us contribute a similar portion of the money needed to fix up the buildings — less, perhaps, the millions in unpaid parking tickets owed by deadbeat diplomats.

There are a number of ways to finance the building, some discussed in that 2001 GAO report. The U.N. could pay for the renovation out of its annual budget, the simplest way to do it. Or United Nations could get traditional, commercial bond financing through the United Nations Development Corporation, the joint city-state agency set up to do just that. The bonds would be paid off out of the dues of United Nations member states, including America.

The United Nations doesn’t want to hear this, and neither, apparently, do the city’s economic development officials, who are said to be backing the secretariat on this point. But insiders tell us they’d better get used to the idea. The American Congress is extremely unlikely to authorize a give-away, on a grand scale, to an international organization it has always viewed with suspicion. If Congress has its way, the buildings might well be left to sink into the ground.

Mayor Bloomberg, who apparently has a special fondness for the U.N., can deliver the wake-up call to Turtle Bay. He can also take a lead in organizing feasible financing for the project that doesn’t rely on soaking the American treasury.