So if I live on top of a mountain, as long as the antenna is no taller than 20 feet above the structure it is attached to, it's legal?
No more than 200 feet tall. Not 20'. Part 95.317
You are not interpreting that section correctly. You may go above 200 feet in height with proper permitting:
In particular, the owner of an antenna structure that is more than 60.96 m (200 ft) in height above ground level (see § 17.7 of this chapter for specific criteria) may be required to notify the FAA and register the antenna structure with the FCC. The 20 foot reference in Part 95.317 is in relation to the proximity to military or public use airports. It is not a general restriction. In the GMRS service, there is a 20 foot restriction for small control and small base stations. Here again, you can gain an exception through proper permitting (in which case it is no longer a "small" station).
- Glenn W9IQ