Under Part 107, a Remote Pilot-in-Command cannot fly an unmanned aircraft higher than 400 feet above ground level (AGL), unless it’s flown within a 400-foot radius of a structure and does not fly higher than 400 feet above the structure’s immediate uppermost limit.

But what constitutes a structure?

We’re so glad you asked! Here’s how the FAA thinks about vertical structures:

https://www.faa.gov/air_traffic/flight_info/aeronav/obst_data/structuretypes/

Hope this helps!