Thought I had an idea for a post but it evaporated; I still don't know why they call that building complex in Bethesda, Maryland the "Air Rights Center," though looked at in the proper light, it's a bit...science-fictional. ("Nope, sorry, sir, you missed the payment deadline. No air rights for the next two weeks -- and no fair trying to hold your breath!")
Doesn't seem to be in in line with the approach for an airport. Old terrestrial microwave path, maybe? Buildings have had to design around them for years and D.C. had plenty. It's a thought. They're certainly not saying on their website. I'm told there's a biking/walking path that runs right under the place!
Update: Per comments, that rails-to-trails path should have been a hint: the buildings were built in the "air rights" over a railway. (Plus enough land for the foundation.) Some people do manage to build real castles in the air! Must've been like working in an old newspaper building when a freight train rumbled through.
4 weeks ago