UPDATE: False alarm. The Royal family did not buy this apartment. It was actually purchased by New Zealand for the country's UN ambassador.
If Queen Elizabeth were to buy property in New York City, which she just did, of course it would be in the Norman Foster-designed tower at 50 United Nations Plaza. TRD reports that the Royal Family just sank $8 million into a three-bedroom, three-and-a-half bathroom pied-a-terre on the 18th floor of the Zeckendorf-developed building. Not only does the building have the perk of being practically on top of the one of the most important sites in the city for dignitaries and foreign affairs staffers, but it also has a personal connection: the Queen famously knighted starchitect Norman Foster in 1990.