400 ft limit

So recently I have heard that drone users are allowed to fly over 400 feet AGL if they are flying within 400 feet of a large structure. (Ex. A user can fly 400 feet over a 300-foot water tower for a total AGL measurement of 700). I was wondering two things: 1) Is this information true? 2) If...

I took off from the top of a cliff and descended over 600 feet to get some shots of the river below. When I tried to climb out of the canyon to bring my Mavic home I could not get it to climb above 400 feet (200 feet from the top of the cliff). Since my battery was running low at that point I...